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.”

The team suspected the “something” wasn’t a rumbling volcano—not in New Jersey—but the cacophony of the giant swarm of cicadas that had just emerged from underground, a population known as Brood X. A colleague suggested reaching out to Jessica Ware, an entomologist and cicada expert at the American Museum of Natural History, to confirm it. “I had been observing the cicadas and had gone around Princeton because we were collecting them for biological samples,” says Ware. “So when Sarper and the team showed that you could actually hear the volume of the cicadas, and it kind of matched their patterns, I was really excited.”

Add insects to the quickly growing list of things DAS can spy on. Thanks to some specialized anatomy, cicadas are the loudest insects on the planet, but all sorts of other six-legged species make a lot of noise, like crickets and grasshoppers. With fiber optic cables, entomologists might have stumbled upon a powerful new way to cheaply and constantly listen in on species—from afar. “Part of the challenge that we face in a time when there’s insect decline is that we still need to collect data about what population sizes are, and what insects are where,” says Ware. “Once we are able to familiarize ourselves with what’s possible with this type of remote sensing, I think we can be really creative.”

DAS is all about vibrations, whether they be the sounds of a singing brood of cicadas or the shifting of a geologic fault. Fiber optic cables transmit information, like high-speed internet, by firing pulses of light. Scientists can use an interrogator device to shine a laser down a cable and then analyze the tiny amounts of light that bounce back to the source. Because the speed of light is a known constant, they can pinpoint where along the cable a given disturbance happens: If something jostles the cable 100 feet down, the light will take slightly longer to return to the interrogator than something that happens at 50 feet. “Every 1 meter of fiber, more or less, we can turn it into a kind of microphone,” says Ozharar.

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