How does the internet cross the ocean?
Not by satellite. The short answer is that almost all intercontinental internet traffic travels through fibre optic cables lying on the seabed, and has done since the 1990s. Satellites carry a small fraction of global traffic, mostly to places cable cannot reach.
A message from Europe to America crosses the Atlantic as pulses of light inside a glass strand about the width of a human hair.
What the cables actually look like
In the deep ocean, a submarine cable is roughly the thickness of a garden hose. Inside it sits a small bundle of glass fibres, surrounded by a steel strength member, a copper conductor, and a waterproof polyethylene sheath. Near the shore, where anchors and fishing gear are a real threat, the same cable is wrapped in heavy steel armour and buried in the seabed by a plough towed behind the cable ship.
How light gets across an ocean
Your data becomes pulses of laser light travelling down the glass. Light loses strength over distance, so the signal has to be amplified along the way. Every 50 to 100 kilometres or so, the cable contains a repeater: a sealed unit that boosts the light without converting it back into electricity. Those repeaters are powered by a high voltage direct current fed down the copper conductor from landing stations on each coast.
That is why the copper is there. The glass carries the message; the copper keeps the amplifiers alive for thousands of kilometres.
Why not satellites?
Two reasons: distance and capacity.
- Distance costs time. A traditional geostationary satellite sits roughly 36,000 km above the equator. A signal has to go up and come back down, which adds hundreds of milliseconds of round-trip delay. A transatlantic cable does the same journey in tens of milliseconds.
- Capacity costs money. A single modern submarine cable system can carry on the order of hundreds of terabits per second. No satellite constellation comes close to that per dollar for bulk long-haul traffic.
Light in glass travels at roughly two thirds of its speed in a vacuum, which is still fast enough that the physical route of the cable, not the technology, becomes the main thing standing between you and a lower ping.
What happens when one breaks
Cables break regularly. Most faults happen in shallow water and most are caused by human activity, principally ship anchors and fishing gear, rather than sharks or sabotage. The internet keeps working because traffic reroutes across other cables automatically.
Repair is unglamorous and slow. A repair ship sails to the fault, drags a grapnel along the seabed to hook the cable, hauls both ends to the surface, splices in a fresh section, tests it, and lowers it back down. Depending on weather and how far the ship has to travel, that can take days or weeks.
The short version
- Intercontinental internet traffic runs on seabed fibre optic cables, not satellites.
- Glass fibres carry the light; a copper conductor powers the amplifiers along the route.
- Amplifiers sit every 50 to 100 km to keep the signal readable.
- Breaks are common, usually caused by anchors and fishing, and traffic reroutes while ships repair them.
Where to check this yourself
Two independent starting points, in keeping with how every Watt & Why episode is built:
- TeleGeography's Submarine Cable Map, which tracks every active and planned cable system in the world.
- The International Cable Protection Committee, which publishes research on cable faults and their causes.