How does a fibre-optic cable no thicker than a garden hose carry roughly 99 per cent of intercontinental internet traffic across 13,000 kilometres of ocean floor?

There are more than 600 active and planned systems in service, and something over 1.5 million kilometres of cable on the seabed. Everything else in the cable, every millimetre of steel and polymer, exists to keep that hair from breaking. In deep water that is the entire cable: perhaps 17 to 20 millimetres across, roughly the thickness of a magic marker. The cable is dressed for the danger it is actually in, which is almost entirely a danger of human origin. On MAREA the arithmetic came out at roughly 20 terabits per second per fibre pair and 160 terabits across all eight.