Enlarge / BUZZ BUZZ BUZZ BUZZ (credit: astrida via Getty Images)

One of the world’s most peculiar test beds stretches above Princeton, New Jersey. It’s a fiber optic cable strung between three utility poles that then runs underground before feeding into an “interrogator.” This device fires a laser through the cable and analyzes the light that bounces back. It can pick up tiny perturbations in that light caused by seismic activity or even loud sounds, like from a passing ambulance. It’s a newfangled technique known as distributed acoustic sensing, or DAS.

Because DAS can track seismicity, other scientists are increasingly using it to monitor earthquakes and volcanic activity. (A buried system is so sensitive, in fact, that it can detect people walking and driving above.) But the scientists in Princeton just stumbled upon a rather … noisier use of the technology. In the spring of 2021, Sarper Ozharar—a physicist at NEC Laboratories, which operates the Princeton test bed—noticed a strange signal in the DAS data. “We realized there were some weird things happening,” says Ozharar. “Something that shouldn’t be there. There was a distinct frequency buzzing everywhere.”