How does the internet cross the oceans?

The internet crosses the oceans the old-fashioned way: through cables lying on the seabed. Submarine cables carry about 99% of the data traffic that crosses the oceans, from your messages to bank transfers and military communications. Satellites handle less than 5% of the world's data, because one cable carries terabits per second while a satellite typically offers around one gigabit per second.

A modern deep-sea cable is only about 25 millimetres across. Inside, thin glass fibres carry your data as pulses of light. The light stays trapped in the core of each fibre by total internal reflection, like a ball bouncing down a pipe that it can never escape. The glass is so clear that the light can travel more than 100 kilometres before a repeater has to boost it, and those repeaters are powered by electricity sent down the cable itself.

The story started with a spectacular failure. The first transatlantic telegraph cable came alive on 16 August 1858. Until then, news between Europe and America travelled by ship and could take weeks. But the cable was agonisingly slow: one character took about two minutes, and Queen Victoria's greeting took 67 minutes to reach Newfoundland. Three weeks later, the electrician Wildman Whitehouse pushed too much voltage into it, trying to speed it up, and destroyed it.

Today's cables face humbler enemies, mostly accidental ones: fishing trawlers, ship anchors, earthquakes and even shark bites. When a cable breaks, a whole country can go dark: in 2022 a volcanic eruption cut Tonga's only cable and isolated the island for several days.