A new fault system on the seafloor was discovered off California's coast by temporarily transforming a pre-existing underwater fiber optic cable into an array of nearly 10,000 seismic sensors, according to a new study. The results showcase the potential of leveraging the extensive web of subsea optical fiber telecommunication cables already spanning the ocean's floor to monitor and record oceanographic and seismic processes in unprecedented detail. Deep below the surface, tectonic forces conspire to fracture and fold the Earth's crust. These rocks break and move at faults. Like geological scars, the surface of Earth is striated with faults; the largest and most lively - where rocks are actively snapping and shifting - are responsible for triggering destructive earthquakes and tsunamis. Mineral deposits like oil and gas are often found along these structures, too. However, charting Earth's fault zones is challenging and many remain unknown, particularly those that lie on the bottom of the ocean. As a result, offshore seismic hazard potential is not fully understood, and information about offshore resources is incomplete. Distributed Acoustic Sensing (DAS), a type of fiber-optic sensing that uses pulses of laser light to continuously detect slight movements along optical fibers, has been used to measure seismic waves and map faults on land. Researchers have suggested it could be used to measure seafloor seismic activity as well. During a maintenance window, Nathanial Lindsey and colleagues temporarily repurposed an undersea fiber-optic cable - part of the Monterey Accelerated Research System - to collect DAS measurements across the continental shelf of California's coast. According to Lindsey et al., DAS turned the decade-old cable into the equivalent of thousands of sensitive seismic sensors. Over the short duration, the authors were able to map a previously unknown fault system and observe several dynamic tidal and storm-driven processes in the water column above. Philippe Jousset discusses the study in a related Perspective.