The answer is optical amplification, wavelength multiplexing, and roughly 600 active submarine cables spanning about 1.4 million kilometres of seafloor — a network built, owned, and repaired mostly by private companies including Google, Meta, and Tata Communications, and carrying, by every serious estimate, more than 99 percent of international data traffic. Satellites handle the rest. The cable itself, in deep water, is about the diameter of a garden hose. Inside it are hair-thin strands of glass that carry pulses of laser light. Those pulses are what your bank transfer, your video call, and your streaming episode of anything actually are, on the leg between continents.
The physics is old. The engineering is not.
The failure state: copper, static, and a delay you could hear
Before glass, there was copper. The first transatlantic telephone cable, TAT-1, went into service in 1956. It could carry 36 simultaneous voice calls between North America and Europe. Thirty-six. For the entire continent. A three-minute call from New York to London cost about $12 in 1956 money — several days of wages for most people. Signals fainted over distance and had to be shouted back to life by electronic repeaters spaced along the line.
Copper had a ceiling and everyone building networks knew it. Every doubling of capacity meant thicker cable, more repeaters, more power pumped in from shore. By the 1970s, engineers were chasing a different idea: send information as light through impossibly clear glass. If the glass was pure enough, a photon could travel kilometres before it lost its shape.
The purity is the part most people never grasp. If seawater were as transparent as modern telecom-grade silica, you could see the bottom of the Mariana Trench from the surface on a sunny day. That is the material lying on the seabed right now.
The turn: TAT-8 and the first optical crossing
In 1988, a consortium led by AT&T, British Telecom, and France Telecom laid TAT-8, the first fibre-optic cable across the Atlantic. It carried 280 megabits per second — roughly 40,000 simultaneous phone calls, more than a thousand times TAT-1’s capacity, on a cable about the same physical size. Cost per call collapsed. The economics of what the internet could become suddenly existed.
Then came the trick that turned a fast pipe into a firehose: wavelength-division multiplexing. Instead of sending one colour of laser light down a fibre, engineers learned to send dozens, then hundreds, of slightly different colours simultaneously, each carrying its own independent data stream. A single hair-thin fibre in a modern cable can now push more than 20 terabits per second. A cable typically bundles 16 to 24 fibre pairs. Do the multiplication and a single cable — the garden-hose bundle — can move well over 300 terabits per second. That is the entire streaming output of Netflix at global peak, several times over, through one cable.
How the pulse actually survives 6,000 kilometres of ocean
Light dims. Even in the purest silica ever manufactured, a photon loses about half its energy every 15 kilometres. Across an ocean, that would leave nothing.
The fix is the erbium-doped fibre amplifier. Every 50 to 80 kilometres along a submarine cable sits a small pressure-housed unit containing a short length of fibre laced with erbium ions. A pump laser excites those ions. When the weakened signal passes through, the ions dump their energy into the signal, boosting it back to strength — without ever converting the light into an electrical signal and back. The pulse stays a pulse the whole way.
Power for those amplifiers is delivered as a constant DC current, up to about 10,000 volts, pumped down a copper conductor sheathed around the fibre bundle from shore stations at either end. If a cable is cut, both landing stations detect the voltage collapse within milliseconds and can pinpoint the break to within about a kilometre using optical time-domain reflectometry — essentially, sending a pulse and timing the echo.
What the cable is actually made of
The core is glass. Around the glass sits a steel tube for crush resistance, wrapped in a copper conductor for power, then layers of steel wire armour, and finally a waterproof polyethylene jacket. In deep water — beyond about 2,000 metres — the cable is lightly armoured and laid directly on the seabed, because at those depths there are no trawlers, no anchors, no realistic threats aside from the occasional undersea landslide.
In shallow water it is a different story. Near coasts, cables are buried one to three metres below the seabed by robotic sea ploughs that ride behind the laying ship, cutting a trench and dropping the cable in as they go. The reason is not sharks. According to a detailed engineering explainer published by News24, shark bites on modern armoured cable are vanishingly rare. The real enemies in shallow water are ship anchors and bottom trawlers dragging steel across the seabed. Most cable faults come from fishing gear or anchors.
Who owns it, who fixes it, who never sleeps
The ownership map has quietly shifted. For most of the 20th century, submarine cables were built by consortia of national telecoms — the same companies that ran your landline. Today, the biggest builders are the hyperscalers. Google has stakes in more than 30 cables. Meta co-owns 2Africa, which at roughly 45,000 kilometres is the longest submarine cable ever built, linking 33 countries around the African continent, Europe, and the Middle East. Amazon and Microsoft are close behind. Tata Communications operates one of the largest independent global networks.
Repair, by contrast, has stayed strangely small. There are only about 60 dedicated cable repair ships in the world. Each is a specialised vessel with a moon pool, dynamic positioning, and grapnels designed to lift a severed cable from four kilometres down. When a cable breaks in the middle of the Pacific, the nearest ship might be three weeks away. The ship steams to the fault, drags a grapnel across the seabed, snags the cable, hauls it up, cuts out the damaged section, splices in a fresh length in a shipboard clean room, tests the join, and lowers the whole thing back down. A single repair takes one to three weeks on station.
The International Telecommunication Union and the US Federal Communications Commission have both moved this year to redraw rules on cable resilience — the ITU targeting the shortage of repair ships and permit delays, the FCC tightening reporting requirements for cable operators serving American landing stations. Japan is subsidising NEC to acquire new cable-laying and repair vessels. Everyone with a coastline is looking at the fleet count and doing the arithmetic.
The Red Sea, the Baltic, and the anchors that keep dragging
In early 2024, three cables in the Red Sea were cut in quick succession, briefly slowing internet traffic between Europe and Asia. The Houthi movement denied responsibility. The most likely culprit, according to subsequent forensic reporting, was the anchor of the abandoned cargo ship Rubymar, which had been drifting after a missile strike. In the Baltic, cables between Sweden, Finland, Estonia, and Germany have been severed multiple times over the past two years by ships whose anchors happened to be down at exactly the wrong moment. Investigators in Helsinki and Stockholm have called it what it looks like: sabotage with plausible deniability.
Writing in The Diplomat, analysts have argued that the seabed is now a contested strategic space — not just for cables but for power links, gas pipelines, offshore wind, and even the first experimental undersea data centres. The piece notes that dragged anchors can serve as cover for deliberate sabotage, making undersea networks vulnerable to political manipulation and coercion. Around Taiwan, the Matsu and Penghu island cables have been severed repeatedly, and local authorities have detained Chinese vessels suspected of deliberate damage.
What happens when a country loses a cable — and who benefits
When Tonga’s single submarine cable was cut by the Hunga Tonga volcanic eruption in January 2022, the country of roughly 100,000 people spent five weeks reduced to a trickle of satellite bandwidth. Banks stopped clearing. Remittances from Tongans working abroad — a substantial share of household income — could not arrive. The economic hit was estimated in the tens of millions of dollars for a single cable break in one small nation.
Silicon Canals has previously explored the political economy of internet shutdowns — how the losses and, occasionally, the gains from a country going dark are distributed. Cable cuts sit at the involuntary end of that same spectrum. The difference is that no minister has to sign the order.
The parts that are still unresolved
Redundancy is uneven. Western Europe and the US East Coast have dozens of cables coming ashore; a single fault causes a rerouting most users never notice. West Africa, small island states in the Pacific, and parts of the Arctic remain single-cable dependent. Cable landing stations — the buildings on shore where fibre meets the terrestrial network — cluster in a handful of chokepoints: Marseille, Fortaleza, Mumbai, Singapore, Fujairah. Attack the building and you do not need to touch the ocean.
Repair ship capacity is the other quiet problem. The global fleet is ageing. Several vessels were built in the 1980s. Training a splicer — someone who can join two ends of a submarine cable in a rolling ship’s clean room and get the light through — takes years. The industry is small, specialised, and largely invisible until something goes wrong.
Silicon Canals has written about other infrastructure stories where the machinery outlasted the empires that built it. Undersea cable is the opposite: a system that most people never picture, running underneath the ocean floor, in glass so pure it barely exists, moved and repaired by a fleet you could count on your fingers and toes. It works. Almost all the time. Every message you send abroad tonight will pass through it. Somewhere off the coast of Cornwall or Alexandria or the Cape, a laser is being pumped through erbium, and a pulse is being lifted back to life for the eightieth time, on its way to a landing station where the light finally becomes electricity again, and then, at last, becomes you.
