Some 70% of Earth’s surface is covered by water, and yet nearly all earthquake detectors are on land. Aside from some expensive battery-powered sensors dropped to the sea floor and later retrieved, and a few arrays of near-shore detectors connected to land, seismologists have no way of monitoring the quakes that ripple through the sea floor and sometimes create tsunamis. Now, a technique described online in Science this week promises to take advantage of more than 1 million kilometers of fiber optic cables that crisscross the ocean floors and carry the world’s internet and telecom traffic. By looking for tiny changes in an optical signal running along the cable, scientists can detect and potentially locate earthquakes. The technique requires little more than lasers at each end of the cable and access to a small portion of the cable’s bandwidth. Crucially, it requires no modification to the cable itself and does not interfere with its everyday use.