The amplifiers under the ocean are powered from shore

Repeaters on a submarine cable, sitting in the dark every fifty kilometres or so, are fed by direct current sent down a copper conductor from the landing stations. On the longest routes the voltage reaches the order of ten thousand volts.

A fibre loses signal, slowly, and across an ocean ‘slowly’ still means the light would be gone without help. Every span, an amplifier — a short length of doped fibre, pumped by a laser — boosts it. Those pump lasers need power. The cable carries a copper conductor for exactly this. The landing station drives a roughly constant current down the line. The repeaters sit in series, each taking a small voltage drop, and the sea is the return path. For a cable of many thousands of kilometres the voltage at the station is thousands of volts, and on the longest systems it reaches around ten thousand or more.

The repeaters are sealed for twenty-five years because nobody wants to haul them up. Their job is dull on purpose. A failed pump in mid-ocean is a ship. The glamour of the light depends on a power supply that looks, electrically, like a long string of devices plugged into a very serious outlet on land.