For a month this spring, Melon was truly living up to her name. She had always been a round cat but never quite this gourd-like—small neck billowing out to a majestic corpulence. This sudden plump came with an attitude. She had abandoned her sweet old habits: sitting on the toilet seat to watch me shower, running back and forth across my apartment until she panted, and leaping on my back to scope out unreachable shelves. I chalked this up to the fact that Melon, now 4 years old, was growing up. Maybe our close bond, which had always felt ineffable to me in ways I cannot articulate without sounding like a TikTok cat mystic, was not actually special, but rather the relationship of a child who needed her parent until, one day, she didn't.

There were other warning signs. Melon had become less and less interested in eating. But she'd always been picky, and I had gone through some drastic life changes—a breakup that came with a division of our two cats—to which I assumed she was still adjusting. Her mane looked terrible, limp and stringy and unkempt, but Melon had never been fastidious about her personal grooming. She'd gotten her annual checkup a few months earlier and left with a clean bill of health. Besides, I was busy, distracted by said life changes and preparing for a vacation and a month off work to write a book. I dropped Melon off at my ex's place, splashed around in Puerto Rico, then came home and picked her up. Neither of us had any idea that anything was wrong. Later that week, my friend Elaine, a fellow cat freak, watched as Melon turned her nose up from a fresh can of wet food. "You know," she said, "we learned Hamlet had cancer when he stopped eating."

My mind began to spin. Hamlet was older when he got cancer, I told myself. Maybe her hunger strike was her acting out for me going on vacation. Friday night, Melon wobbled onto the bed with me and narrowed her eyes into slits. I lay down next to her and studied her face. I tossed, turned, and then spirited us to the emergency animal hospital, empty at 1 a.m. They whisked Melon to the back and I read a book, anticipating the relief I would feel when they would send us home soon with an expensive but clean bill of health. But then I finished my book, and started another. By 3 a.m., I had become too worried to read. I paced my waiting-room cell, decorated only with an ominous daguerreotype of a tuxedo cat.

Recently my family gave me a hologram-making kit that they found at an antique mall in Indiana. I was with them when they first spotted it among the alien masks and vintage comics, and when they pointed it out, my first thought was "yeah right there's no way that's real holograms" followed by "oh wow it looks like actual real holograms".

As a laser scientist this was basically the perfect gift for me, and they sneaked back to the antique mall to pick it up. Fortunately for me, the antique mall hologram kit was not all that antique, just a few years old, and the film was still good. The company Litiholo in fact still makes these kits.

The kit included a red laser, and a bunch of plastic holders that pieced together to hold the film and the laser in the right positions. A little rectangle the size of a deck of cards indicated where I could put the 3D object that I wanted to turn into a hologram - they supplied some dice so my first hologram would be of something that would probably work. For example, they recommend not trying to make a hologram of plants because they move too much (!) during the five-minute exposures.

I've made holograms in the lab with fancy lasers and floating tables and climate-controlled rooms, so I took their instructions seriously when they told me to sit absolutely still and silent during the hologram exposures. The cats were not allowed in the room.

The key to the hologram setup is that half of the laser beam hits the film directly, while the other half bounces off the dice and then hits the film. When the two halves interfere with each other at the film, they embed the film with information about the shape of the light bouncing off the dice. Then, when I remove the dice, the film recreates what the dice was doing to its half of the laser beam, as if the dice were still there.

The result is a hologram that changes depending on what angle you look at it, just like you were looking around the edges and over the tops of the original dice. For this video I scooted the dice out from behind the film so now there's nothing back there at all.

You can really see that in this video of some cool calcite crystals; the camera swings up behind the film to show that the calcite isn't there anymore.

Obviously I haven't perfected the technique yet (maybe the film is a little old after all), but I'm having fun experimenting with making holograms. What thing (that's about the size of a deck of cards or smaller) would you want to see a hologram made out of?

Introduction

As of now (late 2026) LLM (large language model) technology providers, users, and work products flood the public commons. It therefore makes sense to have even a primitive mechanistic mental model of these technologies. You are forced to have an opinion. Without a mechanism or model one tends to fall into disempowering anthropomorphic language. Some clear thoughts on this can be found here and here.

In this note I would like to try and outline a (very) simplified mental model of LLM mechanics and mechanisms. By “mental model” I mean a cartoon to work through in your mind, not a model of the LLMs as having their own mind. I won’t be teaching the history of LLMs, how to build them, how to use them, or their moral or philosophic implications. I will only try to give a very rough outline how the current (2026) LLMs work.

The mental model I would like to bring you to is:

• LLMs are implemented as a flow of numeric signals from a limited number of “attention heads” to output text.
• LLMs are used to realize text transformation and text construction/fabrication. In particular they are approximate plausible “un-censoring” or “un-deletion” functions.

This note will try to make the above two points concrete and clear. After that I will use the model to drive some discussion/speculation.

Here is an example to explain the type of analogy I am hoping to deliver. A common useful mental model for an internal combustion engine car is: it combines air and fuel to produce motive torque, waste heat, and potentially toxic exhaust. This isn’t enough to build a car, but is enough to tell you not to idle one in an enclosed space.

Caveat

I am assuming the current private frontier LLMs are architecturally similar to GPT-4 (2023), but larger and with improvements. So this note is valid for at most such models.

LLM Implementation

Current LLMs take their form from many engineering decisions, three of the most important (in my opinion) being:

• Large scale use of artificial “neural networks” (also called deep learning architecture or connectionist architecture). I am going to take the Bender/Inie advice and try to use the non-standard, but much less loaded, term “weighted network.”
• Clever use of a “un-censor the missing word” training procedures (which led to useful tools such as embeddings).
• Use of bookmark like structures called “attention.” The primitive components of a weighted network do not actually pay “attention” to anything. Instead they imply weights and selective routing of intermediate values. To not lose this distinction, I will use the term “attention heads” instead of “attention.”

We will describe each of these in turn.

Weighted networks

LLMs are implemented in terms of weighted networks. A weighted network is a representation of nested mathematical expressions or formulae, and not a faithful representation of biology. The earliest weighted networks took a number of input signals (say numbers, or voltages) and combined them into one or more output signals. We can imaging the mechanism as in the following diagram.

In this diagram each of the first three volt-meters (v1, v2, v3) represents input signal values, say the length, width, and height of a box. The three knobs (w1, w2, w3; called “weights” or “parameters”) represent how much of each input signal is passed along the arrows to the next node. The complicated vacuum tube amplifier represents a small function of the inputs: in this case 1/(1 + exp(w1 v1 + w2 v2 + w3 v3)). This weighted network converts 3 input values into one output value (itself represented by the last output meter). These nets are not usually realized using electrical components, but as software specifying calculations in GPUs, TPUs, or NPUs.

The knob settings are called the parameters or weights. The knob-settings are picked in a processed called “training” where we adjust the knobs until the weighted network’s outputs are very close to specified results for a great number of training examples. A training example is a pair of an input example (in this case 3 numbers) and the desired output (in this case one number). Even for very restricted topologies, training can be difficult. For the right choice of the weights (knob settings) this circuit may imitate enough of the example input/output pairs to approximate a useful function. However, researchers have been able to implement and train variations of these networks since the 1950s (ref). It is rumored to have cost around $100 million to train GPT-4 (ref). Current LLMs are much larger and more expensive than that.

The choice of the layout of the circuit is called the “topology” of the weighted network. Our small net’s topology has a feature typical to weighted networks: we can sort the nodes into layers and each layer only connects to later layers (never back or laterally). Our example weighted network consists of one node in one layer with 3 knobs or weights. GPT-4 (the state of the art back in 2023) was thought to have millions of input nodes, possibly billions of intermediate nodes, tens of thousands of output nodes, and about 1.8 trillion weights (knobs or parameters) arranged in possibly 120 layers (ref).

Some things to notice is: even a large weighted network is much weaker than a cheap computer.

• It has no scratch-pad or short-term memory. When we change its inputs, its output changes independent of where the inputs used to be. Patterns from the training data determine the weights (or knob-settings), but once training is over the knobs are not moved again.
• No outputs are routed back to earlier portions of the network. This means the calculation can not repeat or iterate steps.

One could implement variations that don’t have the above shortcomings (such as recurrent weighted networks, or trying online reinforcement learning ideas). However current LLMs are thought not to depend heavily on such techniques, as they make training much more expensive for little realized benefit. If a weighted network were modeling biology it would have to have features like the above (as biological neurons are not strictly in layers, and do seem to carry mutable state), but the current known engineering trade-offs are against such features so they tend not to be used.

Clever un-censor training

One hot encoding- the un-clever step

LLMs are demonstrated processing text, not values or numbers as our earlier weighted network did. Researchers adapt weighted networks to text with a very brutal idea called “one hot encoding.” Let’s approach this using an example.

Suppose we with to build a weighted network that works over 9 word utterances from a dictionary of 14 words or tokens. One such utterance is “the quick brown fox jumped over the lazy dog.” To build our adapted net we would build a 9 row (one row for each word in our utterance) by 14 column (one column for each work in our dictionary) paddle-switch array that applies +10 volts where a switch is on and 0 volts where off such as the following.

a

brown

dog

dug

earnest

fox

jumped

lazy

over

quick

red

slow

the

under

the

quick

brown

fox

jumped

over

the

lazy

dog

The voltages from these switches feeds a larger weighted network with many knobs, nodes, and layers. Most of the above cells are switches in the down position. In each row the single switch in the up position is the word the row represents. The property of having one switch on in each row is where the name “one hot encoding” comes from. Current LLMs have an input switch array representing thousands of words over a dictionary of tens of thousands of tokens.

The above representation is a minimal idea that works. It is incredibly inefficient- taking tens of thousands of input switches (or voltages) to represent a single word. It is fairly rigid as the exact positions of the switches are used to encode the words, disallowing ideas such as using different sets of switches for different regions of the input document. And it understands nothing: two rows are either identical (have the same switch up) or disagree in exactly two columns (there is at this point no notion of similarity or synonyms).

Clever un-censorship

Now we are ready for the clever bit. I first saw this in an important research paper introducing a neat text embedding (defined later) called word2vec. The clever idea is the following.

Take our switch array and turn off all of the switches in one row. In this case we have suppressed the fourth row which used to encode “fox.” We wil treat the voltages implied by the new switch array as a single training input.

a

brown

dog

dug

earnest

fox

jumped

lazy

over

quick

red

slow

the

under

the

quick

brown

?

jumped

over

the

lazy

dog

The weighted network will produce an output of meter readings as a function of the input (given above) and the positions of the weight/parameter knots. Here is one possible output.

a

brown

dog

dug

earnest

fox

jumped

lazy

over

quick

red

slow

the

under

Now we construct a single row switch array encoding the missing word (in this case “fox”). Treat the voltages from this switch array as the desired training output.

a

brown

dog

dug

earnest

fox

jumped

lazy

over

quick

red

slow

the

under

Training is just jittering the weight/parameter knobs a small bit so that the result meter needles for this input are closer to zero in the wrong words (down switch positions) and 10 volts in the target word (up switch position). The more serious term for this is stochastic gradient descent. What is amazing is small improvements can accumulate, instead of canceling each other out as we move from training example to example. The training procedure embodies the lesson of the LLM methodology: harvest an unreasonable number of small improvements to yield an approximation of a seemingly impossible desired outcome.

This one sentence could in fact give us 9 training examples- as we cycle through which word-position we wish to un-censor. We use these training examples and many others to train up a weighted network that simulates un-censoring a single word out of sentences! Obviously the simulation can’t be perfect (censorship loses information), but the weighted network settings route signal to plausible replacement word positions. Training is about appropriateness (using only sensible utterances as training data, so there are word relations to learn) and scale (having a lot of training data, for commercial LLMs: most of the web, must help/discussion forums, most social media, most books and periodicals, most technical and scientific papers).

After training is finished we use the weighted network as before on inputs. However we decode these outputs by picking a highest indicating meter as the plausible answer (in this case the 6th meter, which is +10v at “fox”) and get the following one-hot style result.

a

brown

dog

dug

earnest

fox

jumped

lazy

over

quick

red

slow

the

under

Note: the word2vec paper popularized an additional concept of a semantic embedding. One of the layers of the word2vec weighted network was restricted to be only 300 nodes, much smaller than the input or output layers. This “constriction” layer tends to result in trained networks where similar meaning words yield similar voltage patters at the intermediate layer (and different meaning words induce very different voltage patterns). This is in contrast to the original one-hot encoding where different words always disagree in exactly two positions (so there is no useful notion of similar or dissimilar). Embeddings went on to be a core idea and produce additional products such as semantic databases.

The clever training method has given us a new encoding of words- where words that can be used in similar places tend to get similar numeric representations. We are now ready for the last of the big ideas: attention heads.

Attention heads

In my opinion the final component that explains current LLM performance is “attention”, a term defined in the paper “Attention is All You Need.” Attention is a bit anthropomorphic, so I will call them attention heads; think of them as signal routers, position pointers, or text bookmarks.

Current LLMs treat all of your input instructions, the input text, and currently generated output text as encoded inputs as we described above. At first there is no output, so only the system prompts and user inputs are encoded. Then highest LLM-scored word is chosen as the next output word. This process is then repeated by the LLM operator to generate the output text one word at a time in order. By our analogy we are pretending there is a pre-existing plausible output but it has been censored or hidden and the LLM operator is un-censoring an approximation of the output one word at a time. This repetition isn’t part of the LLM, it is supplied by the operator serving the LLM results.

“Attention heads” are just bookmarks that point to positions in the input (and also partially generated output) and also to earlier sections of the weighted network. The positions of the heads are a function of the inputs, so they can appear to move between re-applications of the LLM. This allows effects such as generating words or tokens to complete a sentence (by having a head point into the uncompleted sentence until an ending token is generated), starting or stopping a paragraph, using the same name, and many other seemingly meaningful long-range text interactions. It even can allow semantic effects such as making a point only once, by not pointing the attention head at a given input question if there is what appears to be a related answer already in the partial output. This is likely why LLMs appear to follow instructions- they leave attention heads in the instruction region of the text input.

It is likely the amount of training material and training time is rises very fast in the number of attention heads (probably even exponentially fast). So attention heads are likely expensive even for the rich. I believe they will be a limiting feature of LLM output for a while.

Conclusions/Speculation

I hope you now can envision LLMs as transformations on text realized as a very brutal encoding of input and partial-output text into a flow of numeric signals. The LLM server iterates a “plausible next word” process to generate a sequence of output tokens, as the LLM itself doesn’t implement repetition or iteration. The bookmarks or attention heads help prevent this process from drifting too fast to text unrelated to the original inputs.

I believe features of LLM text (good, bad, and amazing) can be explained in terms of word clustering, approximate un-censorship, and bookmarks. That is we are not forced to explain observed texts in terms of goals, desire, intent, and memory.

LLMs are largely a triumph of scale (size of net, number of knobs/parameters, size and diversity of training data). The LLM weighted networks are of previously unimaginable size. The LLM training corpus can be effectively many times larger than the sum of all written text, as the un-censor training procedure can build many examples from a given text. And LLMs can show amazing results on tasks that don’t need too many attention heads such as specializing or translating a description of a mathematical or engineering technique from the LLM training data into a specific problem application at query time. However, current LLMs have poor performance on seemingly simple tasks that burn attention heads: such as my example of failing at uniquely sorting words.

Unless you are taking the trouble to run a local model, you never directly observe LLM behavior independent of the infrastructure and staff of the service provider. It is hard to tell what is LLM behavior, and what is cached-results or additional custom tools or services. For example LLMs themselves can’t repeat or iterate, however LLM service results are usually the result of iterating “generate next plausible word.” What one is seeing is the result of the LLM plus the service stack.

I’d invite you to try to apply the above (simplified, so not fully realistic) model of LLMs to try and think on a number of scenarios. What are your opinions on the likely outcomes and results of the following thought experiments>

• We take a known mathematical fact (say the Pythagorean theorem), and ask the LLM for a proof.
• We take a pre-existing published math paper, present the first half to the LLM and ask that it complete the paper.
• We take a pre-existing published empirical chemistry paper (concentrating on lab work and results), present the first half to the LLM and ask that it complete the paper.
• We take a new non-published empirical chemistry paper (concentrating on lab work and results), present the first half to the LLM and ask that it complete the paper.

Your answer to the last should vary depending if you treat the LLM as an approximate text processor, or as having a tiny chemist and lab trapped somewhere in its clockworks.

Appendices

GPT-4

GPT-4 is partially documented here. In particular (quotes from article):

• “Transformer-based model pre-trained to predict the next token in a document.”
• “GPT models are often trained in two stages. First, they are trained, using a large dataset of text from the Internet, to predict the next word. The models are then fine-tuned with additional data, using an algorithm called reinforcement learning from human feedback (RLHF), to produce outputs that are preferred by human labelers.”

Note we don’t comment on the full complexity of transformers, just the attention head feature.

On Hallucination

From the GPT-4 Technical Report.

GPT-4 has the tendency to “hallucinate,” i.e. “produce content that is nonsensical or untruthful in relation to certain sources.”

I think of this as anthropomorphizing, but the online dictionaries don’t seem to support me:

hallucinate (third-person singular simple present hallucinates, present participle hallucinating, simple past and past participle hallucinated)

• (ambitransitive) To seem to perceive things (with one or more of one’s senses) which are not really present; to have visions; to experience a hallucination.
  Synonyms: imagine, see things
• (artificial intelligence, of a model) To produce information that is not supported by the model’s training data.

Joking aside: for a non-technical audience “hallucinate” evokes perception and mental state, not a mismatch from training data. The LLM output was always a construct or fabrication, even when it matches the truth.

On Recursive Self-Improvement

Investors have been hoping for recursive self-improvement where it turns out one of the things LLMs are good at is suggesting game changing improvements to LLMs (and then trigger some sort of technological singularity). Things are in fact moving very fast in the LLM space. So there are some issues in working out if unfounded confident-sounding advice from an LLM is the best steering for a long and expensive training cycle. Optimizing in the presence of delayed feedback is notoriously treacherous.

Note on Experiments

Note both the word2vec and attention papers are very good experiments in that they deliberately use overly simplified complementary tools and procedures to show the claimed positive effects are from the claimed technology. So not only are the results reproducible, one can do better by swapping in better complementary tools (such as tokenizers, embedding choices, and so on).

Things LLMs should be bad at

Under our mental model LLMs should produce bad results when there is a dominant plausible wrong answer and when there are many relations between bits of the answer to maintain. The ideas being if no answer is plausible the LLM result will likely be equivocal and unconvincing. When there are a lot of relations to maintain in the answer (such as puzzle conditions) then attention heads are used up mapping marking relations between parts of the answer text, moving them off relations to the input text and so-called instructions and question. Another attack is to have an input text that superficially looks like a common puzzle, this way the LLM output tends to be aligned to the related answer.

Some failing examples include:

• Failing to uniquely sort words. This is exploiting the presumably limited number of attention heads.
• Failing on the “surprise the doctor is your Mother, not a man!” puzzle. This is the result essentially being a copy of the answer to a similar puzzle, not the one asked. Note the claimed “reasoning” is better described as “initial tokens.” Notice the so-called reason trace does start with text close to the question and with text at the correct answer, however it is full of weird non-sequitur stops and starts. It is text that plausibly looks like reasoning, probably not reasoning. The ideas of additional state heads and state-reprocessing are in fact good, they just do not necessarily work in the way the author or even I think.
• The “should I walk to the carwash” example. Trick questions often exploit missing context or corner-cases of reasoning. This is not in fact a trick question: for a human going to the car wash almost implies it is to get the car washed. Yes it could be to buy an air-freshener or pick up an already committed car, but those are the exceptional cases. That the LLM suggesting walking is evidence against it using non-textual semantics.

The question isn’t: can we make silly examples LLMs get wrong. It is: are the equivalents of these problems lurking in our important project we delegated to the LLMs?

Image credits

https://commons.wikimedia.org/wiki/File:Mcintosh_MC275_european_version.jpg#/media/File:Mcintosh-MC275-glow.jpg ,
https://hackaday.com/wp-content/uploads/2022/09/AMSAI.png , and the author.

At the northeastern edge of La Línea de la Concepción, on a scrubby Mediterranean beach called El Burgo–Torrenueva, there is an old battlement-tower, La Torre Nueva, and not much else. It was part of the system of coastal watchtowers during the 16th century that would defend the area against the incursion of the Barbary corsairs. The coordinates are 36°12′36″N, 5°19′27″W. Walk the tideline and you would never know that buried two metres beneath the sand, a fibre-optic cable comes out of the sea here and turns into the internet. It’s the start of a line that runs across the Strait of Gibraltar to Ceuta, on the African coast, and on toward two continents. Nearly everything you do online that crosses an ocean passes through a cable like this, ending, in most cases, underneath a similarly unremarkable patch of coast.

When we talk about the internet we do so as if it were air. Ambient, ownerless, and everywhere. In reality, however, it is the exact opposite, because international data doesn’t (normally) travel by, let’s say, satellite, despite what most people might assume. It travels through roughly 1.5 million kilometres of very real (and very owned) fibre-optic cable lying on the seabed, surfacing at a small number of carefully chosen landing points.

For these landing points you normally need a gently sloping seabed, mild currents, and little marine traffic, so that anchors and trawlers don’t sever the line. Suitable spots are scarce enough that the same beach usually becomes the shared landfall for several cable systems at once.

Cables? What cables?

Unlike what you might be thinking of at first, submarine cables aren’t your run-of-the-mill Ethernet or fibre cable. The hardware that does the heavy lifting out in the deep ocean is about as thick as a garden hose with roughly 25mm across and weighing in at around 1.4 tonnes for every kilometre. The part that carries your data is a small bundle of glass fibres, each one around the same thickness as human hair, sitting in the very middle.

Everything else wrapped around those fibres is there to keep them alive in a deeply hostile environment. Working outward from the core, the fibres sit in a water-blocking gel inside a thin copper or aluminium tube, which is sheathed in polycarbonate, then an aluminium water barrier, then a layer of stranded steel wires that give the cable its tensile strength, then a wrap of mylar tape, and finally an outer skin of polyethylene. The copper is for power, because the cable doubles as a very long extension lead, which we will get to in a moment. Closer to shore, where trawlers and anchors roam, the whole thing gets one or two further jackets of galvanised steel armour wire, swelling it to 50mm or more in diameter and several times the weight. Hence, the cable that surfaces on our Spanish beach is buried a couple of metres down and not simply left lying on the sand.

The reason a copper conductor runs the entire length is that light, no matter how pure the glass, slowly fades as it travels, and so every 50 to 80 kilometres the cable is interrupted by a repeater, which is an optical amplifier that boosts the signal back up before passing it along. Each repeater needs electricity, and because the fish sadly still didn’t manage to install power sockets on the ocean floor, the shore stations at either end have to feed a direct current of anywhere between 3,000 and 15,000 volts down that copper core, to literally power the cable from both ends at once.

On top of the amplification, modern systems lean on a stack of clever tricks to keep the signal intelligible across thousands of kilometres of glass, including wavelength-division multiplexing to cram many separate colours of light down a single fibre, coherent detection to read them back out, and forward error correction to repair whatever gets garbled along the way.

Length, then, is mostly a question of power and amplification rather than of the glass itself. Shorter hops can dispense with repeaters entirely, hence an unrepeatered span will happily run to around 250 kilometres on amplifiers at each end alone, which is roughly the length of the line we started this post with. At the other extreme, a single system can stretch across an ocean, and the longest of them, like the 2Africa cable encircling the continent it is named after, run to tens of thousands of kilometres.

Who is laying cables?

The actual manufacturing and laying of these cables is, perhaps a little surprising for something the entire global economy rests on, the business of only a small handful of companies. The bulk of the world’s submarine cable is built and installed by just four suppliers, namely the American SubCom, the French Alcatel Submarine Networks, the Japanese NEC, and the Chinese HMN Technologies. They own and operate the specialised fleet of cable-laying ships, which aren’t exactly the kind of boat you would recognise from a harbour, but more like a purpose-built vessel carrying thousands of kilometres of cable coiled in enormous tanks below deck, rolling it out over the stern at a steady walking pace as they crawl across the ocean.

Deploying a new system is a multi-year effort that begins long before any ship leaves port. First somebody, these days increasingly a content giant rather than a phone company, decides a route is worth having and assembles the money for it, either alone or as a consortium of several owners sharing the bill. Then comes a marine survey, in which a ship maps the intended path along the seabed to find the gentlest, safest route around wrecks, trenches, and other people’s cables, followed by the permitting, which is the paperwork of securing landing rights and concessions from every jurisdiction the cable so much as touches. As we are about to see on the Spanish beach, this can generate a remarkable quantity of bureaucracy.

Only once all that is settled does the cable get manufactured to length, loaded onto the ship, and laid, with the vessel simply lowering it onto the seabed in deep water and a sea plough burying it a metre or two beneath the sediment closer to shore, where the danger from fishing and anchors is greatest. A working ship covers somewhere in the region of 100 to 200 kilometres a day, so an ocean crossing takes several weeks at sea.

A transatlantic system running some 7,000 kilometres typically costs in the order of 250 million USD, while a longer trans-Pacific route can easily climb towards 400 million, and the cable itself runs anywhere from roughly 6,000 to 20,000 dollars per kilometre, depending on how many fibre pairs it carries and how heavily it is armoured. Keep in mind that the spending does not stop once the cable is lit, because a submarine cable has a design life of only around 20 to 25 years and on top of that there are somewhere between 150 and 200 faults occurring across the world’s cables in a typical year. The overwhelming majority of them are not caused by sabotage or sharks, but by the combination of fishing gear and dragged ship anchors. Each break has to be mended by sending out one of a small number of dedicated repair ships, that are on permanent standby under regional maintenance agreements, to grapple the cable up off the seabed, haul both severed ends to the surface, splice them back together, and lower the repaired thing back down. This is slow and weather-dependent work that is quite expensive.

Who owns the cables?

With the data provided by TeleGeography’s Submarine Cable Map I have put together a list of the (co-)owners of undersea cables and sorted it by the number of cables each individual company has a stake in. The full dataset runs to some 473 distinct owners, the overwhelming majority of which are obscure national and regional carriers you will never have heard of, so rather than just dumping the entire list here, I limited it to the hundred most prolific (co-)owners:

What jumps out, at least to me, is the name sitting right at the top. For most of the history of this infrastructure the owners were telephone companies, the _BT_s and _AT&T_s and _NTT_s of the world, laying cables to carry one another’s calls and, later, traffic. Google now has a stake in more submarine cables than any traditional carrier on the planet, with Meta not far behind, and Microsoft and Amazon both slowly accumulating their own share. The companies that fill those cables with traffic have, over the past decade or so, decided that they would rather own the pipes than rent them.

The other thing the numbers tell you is just how long the tail is. Of those 473 owners, some 260 appear on exactly one cable, and more than 340 of them, north of seventy percent, on no more than two. These are the world’s national telecoms, each one buying a slice of the handful of consortium cables that happen to land on its particular stretch of coast, which is also why so many of the big international systems list a dozen or more co-owners apiece. The internet, seen from this angle, is less of a single network and more of a mix of local operators, all chipping in for a share of the same few very expensive ropes across the ocean.

Going back to the beach in Spain

To see what it looks like where the cable actually meets the land, let’s head back to that beach in La Línea.

The cable that surfaces there is called Dos Continentes, it belongs to GTD, a Chilean telecoms group, and it’s a relatively small regional system consisting of two armoured fibre cables looping across the Strait of Gibraltar to Ceuta, the Spanish enclave on the African coast that depended on a single ageing link before this one was built.

I went looking for exactly where it comes ashore, and the paper trail gives an idea about how invisible this infrastructure actually is. The cable lands in Spain, but the public Spanish government map of coastal concessions doesn’t seem to show it, because it looks like coastal permits in Andalusia are devolved to the regional government. The landfall instead shows in a regional registry, in a signed resolution buried under an expediente number. That document pinpoints where the cable enters the public maritime domain, at grid reference X=290,935, Y=4,009,603, just seaward of the beach manhole. The cable then runs inland, buried as the permit insists (“no exterior element above ground level”) to what is presumably a network node, where traffic is fed into GTD’s pre-existing terrestrial dark-fibre network, from where it’ll eventually travel to one of the actual GTD data centres in Madrid, Barcelona, Bilbao/Sopelana, and Sevilla.

On its way out to sea it crosses three older cables already lying on the seabed, namely Europe India Gateway, ATLAS, and FLAG. As can be seen (or, well, actually not) even an empty-looking patch of water off a Spanish beach is layered with other people’s infrastructure.

The Dos Continentes cable (Segment I, La Línea - Ceuta Sur ramal), owned by GTD Cableado de Redes Inteligentes, S.L.U., the Spanish arm of the Chilean GTD group, has a total length of ~105 km and is in service since 2020 under the signed concession resolution from the Junta de Andalucía (Dirección General de Calidad Ambiental y Cambio Climático), expediente CNC02/19/CA/0009, dated 14 January 2020.

The two key points, as given in the resolution’s coordinate table are:

To convert the UTM coordinates I used the official Instituto Geográfico Nacional (IGN) Calculadora Geodésica with the following settings:

• Transformation type: Transformación de Datum
• Reference system: ETRS89
• Input coordinates: UTM
• Huso (zone): 30

ETRS89 and WGS84 differ by only centimetres in practice, so the resulting coordinates (WGS84-equivalent) can be dropped straight into any consumer map or GPS app:

Both points sit on Playa de El Burgo–Torrenueva, beside the Punta de Torrenueva tower, at the northeastern (Levante / Mediterranean-facing) edge of La Línea de la Concepción, against the municipal boundary. The resolution describes the route as passing “muy cerca de la torre-faro existente en la Punta de Torre Nueva”.

As you can see, however, you see nothing. :-) The permit requires the whole installation to be subterranean (“no exterior element above ground level: No manholes, splices, connections or terminals.”), hence you can stand exactly on the landfall, but it’s a point in the sand by a tower, and not a structure. On the afternoon I was there, a couple of dozen people were spread out on that stretch of sand under parasols, probably not even knowing that somewhere underneath them the link that carries an entire enclave’s traffic to another continent came out of the sea.

It is interesting to see that what has changed most over the past decade isn’t the technology itself, but who pays for it. For a century these systems were built by carriers selling capacity to one another, which made the network something close to a shared utility with many owners. Today, however, the largest (co-)owner of submarine cable on the planet is an advertising company. It probably makes sense in their position, however it is a change in how the network is governed, and, more importantly, it seems to have happened almost entirely out of public view, which is worrying.

If you live anywhere near a coast, there is a decent chance one of these things lands within driving distance of you, and the TeleGeography map will get you to roughly the right bay. Getting from there to the actual patch of sand takes some amount of digging through concession resolutions, planning registers, environmental reports, and sometimes the local newspaper archive. It took me an evening of reading to narrow it down, but I can recommend to do this exercise if you’re curious about the world that you’re living in and, more importantly, the hidden infrastructure surrounding you.

PS: Maybe we picked the wrong word and should have called it the trench rather than the cloud?

A reporter for The New York Times, Streitfeld often writes about technology. This article appeared August 18, 2016.

Social media is bad for children.

That was the blunt verdict in a recent spate of high-tech court cases in California, Kentucky and New Mexico. At a moment when Silicon Valley has never been more dominant or ambitious, one of its signature products has all the allure — and the lawsuits — of cigarettes circa 1990.

On Tuesday, opening statements begin in Oakland, Calif., for the latest social media trial. California and other states have brought a case against Meta, owner of Facebook and Instagram, saying the company “harnessed powerful and unprecedented technologies to entice, engage and ultimately ensnare youth and teens.”

Meta denies the allegations. But you know who else has some issues with social media, screen time and the addictive nature of devices? Quite a few of the tech executives in Silicon Valley, at least when it comes to their own households.

“I don’t generally want my kids to be sitting in front of a TV or computer for a long period of time,” Mark Zuckerberg, Meta’s chief executive, once said. Interacting with people is good, but passively consuming content — “just going from video to video” — was not associated with “the same positive benefits,” he explained.

When Mr. Zuckerberg’s second child was born in 2017, he and his wife, Priscilla Chan, wrote in an open letter to her that “it’s important to make time to go outside and play.” Smelling flowers got a shout-out. So did picking up leaves. Nothing digital rated a mention.

Tech executives like Mr. Zuckerberg don’t talk about their own families often. When they do, however, they usually make clear their wariness of tech. For their children, tech does not play the same role it does for the rest of society: teacher, babysitter, all-purpose distraction and, increasingly, constant companion. It seems barely there at all.

The executives’ comments are generally off the cuff and are snapshots of a moment. Mr. Zuckerberg, for instance, talked about his children seven years ago and has said little since. A Meta representative declined to say anything more.

Peter Thiel, who was the first outside investor in Facebook and a longtime board member, said he limited his children — then 3½ and 5 — to “an hour and a half a week” of screen time. When he mentioned this at the Aspen Ideas Festival in 2024, the audience gasped. Many children that age experience that amount of screen time in a day, or even a morning.

But Mr. Thiel said he was a typical Silicon Valley parent.

“There are probably a lot of people in tech who do something quite similar for their own families,” he said, adding pointedly, “There’s some questions that that might lead you to ask.” Mr. Thiel did not respond to emailed questions.

Even as the new Meta case was heading to trial, British researchers raised new alarms about tech and young children.

“The benefit of digital screen exposure for babies is negligible, while the potential risks of use on physical, cognitive, social, and emotional development are substantial,” the interdisciplinary Action on Digital Device Immersive Conditions Team, a group of university academics, said in a May report.

Tech people seemed to realize this from the beginning, said Mara Fath, an artist and bookseller who recalled the influx of programmers into her Upper Haight neighborhood in San Francisco 20 years ago.

“All our neighbors worked for those firms — Facebook, Google and Apple. None of them would let their children interact with any screens,” Ms. Fath said. “They all knew how addicting they were and how to protect their own children.”

Cautions and caveats from tech leaders piled up over the years. “We didn’t give our kids cellphones until they were 14,” Microsoft’s co-founder Bill Gates said. When Sundar Pichai’s son was 11, the Google chief noted he “still doesn’t have a phone,” adding that “our television is not easily accessible.” Apple’s Tim Cook said he would not let his nephew on a social network.

By now, it’s a cultural cliché. In “The Audacity,” Jonathan Glatzer’s new AMC series, a girl at a Silicon Valley party is surprised that a boy has a smartphone. “Arms dealers don’t give their kids land mines,” she explains.

Phones are banned at the Waldorf School of the Peninsula, two miles from Google’s headquarters. “No phones means no distraction, no social media anxiety, and no constant comparison. Just real childhood,” the school explains on its website. Waldorf’s price for a real childhood: as much as $47,500 a year.

Several Waldorf board members are professionally focused on artificial intelligence, but at the school machine learning is minimized in favor of “human learning.” The goal is to train the future elite to think clearly.

“While machines optimize tasks and automate decisions, our graduates develop the uniquely human capacities that will define leadership in 2030 and beyond,” the school says.

Snap’s chief executive, Evan Spiegel, has credited a low-tech home environment for his success in running a high-tech firm. Growing up in the 1990s, Mr. Spiegel wasn’t allowed to watch television until he was “almost a teenager.” He eventually realized that was a good thing. “I spent a lot of time just building stuff and reading,” he said.

Mr. Spiegel aimed to carry on that tradition: only 90 minutes of screen time a week for his 7-year-old, he said in a 2018 interview. He now has four children with his wife, Miranda Kerr.

“For the 2-year-old, it’s like zero screen time,” he said on a podcast in April. “We really just want to essentially read with him and play, explore outside, and those sorts of things.” The 6- and 7-year-olds are “infrequent movie watchers, but other than that again, we don’t give them phones or anything like that.” The 15-year-old, however, is “all-in on technology.”

Steve Chen, a co-founder of YouTube who sold the video company to Google in 2006, told students at Stanford last year that he was worried that his children would end up unable to watch anything longer than 15 minutes.

“The modern digital environment trains kids to expect everything to be summarized into a 280-character tweet or a short-form video clip,” Mr. Chen said in a message. His goal was to make sure his children “know how to do things the slow, hard way.”

With A.I., Silicon Valley’s latest product, tech is less a passive broadcast device than a direct substitute for human interaction. It doesn’t come between the parent and child. It replaces the parent.

Consider a new app that Meta is testing, StoryKit. Your child is demanding you make up a story at bedtime? No problem. The app will take the child’s name and a few details and quickly spin a personalized tale. “No more guilt about screen time,” the app promises.

Last week, Mr. Zuckerberg wrote in an essay that “my 8 year old daughter can already code her ideas and produce videos.” When they bake together, he said, A.I. “orders the ingredients, and then offers suggestions.”

Others are as cautious with A.I. as they were with social media. “I asked my 16-year-old son what I should say to you,” YouTube’s Mr. Chen wrote. “He responded, ‘Please tell them that I read books and watch movies and that I dislike A.I.’”

Sam Altman, the chief executive of OpenAI, has his own “cool use case” for A.I. and kids. He recently suggested on X connecting family calendars to ChatGPT. On the drive to school, ChatGPT could “make a podcast that talks about one kid’s soccer game that afternoon, one kid’s upcoming birthday, some news, etc.,” he posted.

Using A.I. to talk to your family is something for others to embrace, though. As a new father, Mr. Altman finds his own attitude toward technology becoming more resistant.

A thing that has changed hugely is how I feel about algorithmic feeds and iPads in small children’s hands,” he said in April on the podcast “Mostly Human With Laurie Segall.” As for A.I. and his son, “I don’t know when I would let him talk to A.I., but I’d rather be on the late end of what’s reasonable there, not the early end.”

Like other tech executives, Mr. Altman said he was often bored as a child, which at the time he hated. His new view is that boredom was “super valuable in all of these strange ways.” He said he looked forward to sending his child outside to play in the dirt.