The answer is that the cable is barely doing the work. The glass inside it is. Every intercontinental fibre carries dozens of separate laser wavelengths at once, a technique called wavelength-division multiplexing, each wavelength encoded in both the brightness and the phase of the light and split across two polarisations, then re-brightened every 60 to 100 kilometres by erbium-doped amplifiers sealed inside torpedo-shaped repeaters. Stack those tricks and eight pairs of hair-thin fibres will move 160 terabits per second, which is what the trans-Atlantic MAREA cable was rated for when Microsoft, Facebook and Telxius finished laying it between Virginia Beach and Bilbao in 2017. The whole assembly is about as wide as a garden hose. Satellites, meanwhile, are a rounding error: the US Federal Communications Commission puts them at 0.37 per cent of American international capacity, a figure TeleGeography cites when it explains why nobody seriously proposes replacing the cables.
Inside that hose, most of the volume is not communicating at all. It is armour.
There are more than 600 active and planned systems in service, and something over 1.5 million kilometres of cable on the seabed. Some routes are trivially short. Others were monsters: SEA-ME-WE 3 ran roughly 39,000 kilometres from Norden in Germany to Okinawa in Japan by way of the Suez chokepoint, and held the record for longest cable on Earth until it was retired in December 2024. Long or short, they share the same modest cross-section.
What is actually inside the hose
A deep-sea cable is a nested cylinder, and the order of the layers is the whole engineering argument in miniature.
At the centre sits a bundle of optical fibres, typically eight to thirty-two of them, arranged in pairs so that each direction of traffic gets its own strand. Around the bundle is a steel or copper tube packed with a water-blocking gel. Around that, a copper conductor carries several thousand volts of direct current. Then a polycarbonate insulator, then stranded galvanised steel wires for tensile strength, then an outer sheath of polyethylene.
The fibre itself is where the scale becomes hard to hold in the head. The light-carrying core is about nine micrometres across, roughly the width of a single red blood cell. The pure silica cladding wrapped around it brings the strand to 125 micrometres, which is about the width of a human hair. Everything else in the cable, every millimetre of steel and polymer, exists to keep that hair from breaking.
In deep water that is the entire cable: perhaps 17 to 20 millimetres across, roughly the thickness of a magic marker. Across the abyssal plain, three or four kilometres down, where nothing much happens except the slow settling of sediment, the thing really is no fatter than something you would use to water a lawn.
Nearer the coast it fattens fast. In water shallow enough for trawl doors and anchor flukes, one or two extra layers of heavy steel armour wire go on, and the shore end can be as thick as a soft drink can. The cable is dressed for the danger it is actually in, which is almost entirely a danger of human origin.
Why light is such a good courier
When a laser at one end of a fibre fires, the pulse travels down the core by total internal reflection, striking the boundary with the slightly less dense cladding at a shallow enough angle that it bounces rather than escapes. The light is not so much guided as trapped.
What makes the trick worth doing over an ocean is the transparency of the glass. At wavelengths near 1550 nanometres, in the near-infrared, modern silica is so clear that a signal can run tens of kilometres before it fades to the point of needing help. A block of it kilometres thick would still be see-through.
That window at 1550 nanometres is not an aesthetic choice. It is the wavelength at which silica happens to absorb least, and essentially the entire long-haul telecommunications industry is built on top of that accident of chemistry.
How dozens of colours share one strand
A single laser modulated as fast as electronics allow gets you tens of gigabits per second. Getting to tens of terabits means sending many signals down the same strand at once without letting them interfere.
Wavelength-division multiplexing does this by assigning each signal its own colour of infrared, dozens or even a hundred of them sharing one fibre in the way radio stations share the air. Each of those colours is then modulated coherently, with information encoded in the amplitude and the phase of the wave and carried on two perpendicular polarisations simultaneously. A receiver that can track phase can pull four or more bits out of every symbol instead of one.
On MAREA the arithmetic came out at roughly 20 terabits per second per fibre pair and 160 terabits across all eight. Microsoft put the full-cable figure at enough to stream 71 million high-definition videos at the same time, in a bundle weighing 4.65 million kilograms and lying more than 5,000 metres down.
Design capacity is a floor, not a ceiling. Because the fibres never change but the equipment on the beach does, operators upgrade cables by swapping terminal gear rather than relaying steel. TeleGeography now lists MAREA’s potential capacity at 224 terabits per second, forty per cent above what it was commissioned at, on exactly the same glass.
The newer systems went wider rather than harder. Google’s Grace Hopper cable, live between New York, Bude in Cornwall and Bilbao since September 2022, carries 16 fibre pairs for a design capacity of about 352 terabits per second. Meta’s 2Africa, whose core was completed in November 2025, uses the same 16-pair spatial-division approach across 45,000 kilometres and 33 countries. Running more moderate lasers down more fibres turns out to beat running heroic lasers down few, largely because the power budget at the bottom of the sea is finite.
The repeaters, and the power line that feeds them
However clear the glass, light attenuates. Every 60 to 100 kilometres the cable passes through a repeater, a pressure-rated cylinder about a metre long with the silhouette of a small torpedo.
Inside is an erbium-doped fibre amplifier: a short coil of fibre seeded with erbium ions which, when excited by a pump laser, dumps their energy into the passing signal. Nothing is converted to electricity, decoded or regenerated. The photons simply get louder and keep going, which is why a repeater built in 1998 can still amplify a modulation scheme invented in 2015.
Feeding those repeaters is its own quiet feat. The copper conductor wrapped around the fibre bundle carries direct current at up to about 10,000 volts, pushed from power-feed equipment in buildings at both landing points, with the seawater itself completing the circuit through sea-earth electrodes. A trans-Pacific system may have well over a hundred repeaters hanging off a single continuous line thousands of kilometres long, and the whole chain has to survive twenty-five years without a service call.
Laying it is slow work. Specialised ships, such as the fleet operated by Alcatel Submarine Networks, pay cable out over the stern at perhaps 100 to 200 kilometres a day, ploughing it into the seabed where the water is shallow and simply letting it settle where it is not. A single ocean crossing can occupy a ship for months.
Why the hose survives the abyss
The deep ocean is, counter-intuitively, a gentle place. Past the continental shelf there are no trawlers, no anchors, almost no current and very little temperature swing. The cable is not fighting anything down there.
The damage happens in the shallows. Roughly 200 faults occur worldwide each year, and International Cable Protection Committee data attributes about two-thirds of them to fishing gear and dragged anchors, with earthquakes, turbidity currents and component failure accounting for most of the rest.
Sharks make better headlines than they do statistics. In the ICPC’s records, fish bites of any kind accounted for zero cable faults between 2007 and 2014. The animal most dangerous to the internet is a person on a boat.
Repair follows a procedure that would be recognisable to a Victorian cable engineer. A ship steams to the fault, drops a grapnel, hauls the severed ends to the surface one at a time, splices in a fresh section and lowers the loop back down. Two weeks is a normal turnaround; permits and weather routinely make it longer.
Redundancy is what makes any single break survivable, and its absence is what makes a break catastrophic. When the Hunga Tonga-Hunga Ha’apai eruption severed Tonga’s only international cable on 15 January 2022, there was nothing to reroute to. Cloudflare’s traffic data shows the country effectively offline for 38 days, with a trickle of satellite traffic standing in for a nation, until the repair ship Reliance replaced a 92-kilometre section. Countries with a dozen landings never notice a cut. Countries with one notice nothing else.
The same web of dependencies is why a DNS lookup for a foreign website can finish in under a hundred milliseconds despite crossing an ocean, and why a single mistyped command in the wrong console can take out more of the internet than any fishing trawler ever has.
Why not satellites
Every generation asks why the world does not simply put the internet in orbit, and the answer keeps coming back the same: capacity and latency, in that order.
A geostationary satellite parks about 36,000 kilometres up. Light needs roughly half a second to make the round trip, before any equipment touches the signal, and the throughput on offer is measured in gigabits rather than terabits. Low-Earth-orbit constellations such as Starlink fly a few hundred kilometres up and cut the delay to tens of milliseconds, which is genuinely transformative for a remote village and still nowhere near the bulk capacity of one fibre pair, let alone sixteen of them.
There is a historical rhyme in this. The first trans-Atlantic telephone cable opened in 1956 and carried a few dozen simultaneous calls. Telstar went up six years later and looked, briefly, like the future. Seventy years on, the cable won, and it won by getting narrower and cleverer rather than bigger.
So the intercontinental internet remains a physical object: pulses of infrared threaded through strands of glass finer than a hair, sheathed in steel and polymer no thicker than a garden hose, lying in the cold and the dark four kilometres beneath the shipping lanes. It carries very nearly everything humanity says to itself across the oceans. And tonight, as it has every night since it was laid, nobody using it will give it a thought.