Researchers piggybacked on a fiber-optic telecommunications cable to sense seismic activity in Monterey Bay, turning 20 kilometers of cable (in pink) into the equivalent of 10,000 seismic stations along the ocean floor. The cable is normally used to communicate with an off-shore science node (the Monterey Accelerated Research System, or MARS). During a four-day test, scientists detected a magnitude 3.5 earthquake 45 kilometers away in Gilroy, California, and mapped previously uncharted fault zones (yellow circle). Illustration by Nate Lindsey

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