Satellites get the credit. Almost all of it actually crosses the ocean floor inside a tube you could hold in one hand. Here is what is down there.
More than 600 cable systems are in service, totalling roughly 1.5 million km of cable. Between them they carry about 99% of the world's intercontinental data.
Cut one open and it is mostly protection: steel armour wires, polyethylene, waterproofing. All of it exists to shield a core no thicker than a pencil.
The working element is a handful of glass fibres, each thinner than a human hair. A modern cable runs a dozen or more pairs and moves hundreds of terabits a second.
A cable ship pays it out over the stern for weeks at a time. Near the coast a seabed plough buries it; in deep water it simply lies on the mud.
Each end surfaces at a landing station: an unmarked, heavily powered building near a beach. From there the traffic joins the ordinary land network.
The classic villain is a myth. The real one is a ship's anchor dragging across the seabed — on its own the single biggest cause of cable faults worldwide.
Add bottom trawling and the two together account for roughly 85% of faults. Across the whole network that works out at around 200 breaks every year.
Engineers fire a light pulse and time the reflection to locate the fault. A repair ship then drags a grapnel hook until it snags the cable and splices it on deck.
A transatlantic telegraph cable first spoke in 1858 and died within weeks. In 1866 the giant steamship Great Eastern laid one that lasted — the same route logic we still use.
Light in glass moves about two-thirds of its vacuum speed, so New York to London and back lands under 80 milliseconds — through a cable lying in the dark.