The planet-spanning web of undersea internet cables acts like a giant ear. The glass thread inside responds to water movements: currents, tides, and distant earthquakes bend the cable. Light traveling along the thread takes slightly longer: this delay is measured on shore against a reference. That’s how the speed of light reveals itself.

Engineers tested the idea on the Ireland–Iceland cable (1,770 km), sending a microwave signal instead of a laser. Over four months, instruments recorded the rhythm of tides, the footsteps of storms, and the jolts of distant earthquakes. The same principle is used in gravitational wave detectors — except here they catch not the trembling of space, but the sway of the ocean.

The most surprising part: the cable stayed online and kept carrying internet traffic, without interfering with the measurements at all. This vast network already lying on the seabed becomes a single ear for the ocean at no extra cost.