Whether it’s under a city or on top of a remote glacier, an optical cable will wiggle when disturbed — for instance, by the motion of traffic or of seismic waves. Distributed acoustic sensing, or DAS, captures those tiny movements. Laser light pulses are sent out from the interrogator into the fiber. As they travel, some photons hit defects in the fiber, which scatters them, and some of this scattered light makes it back to the source. Analyzing this “backscattered pulse” and comparing it with the light that was originally sent out allows researchers to detect environmental events.