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.

Ozharar’s team focused on a loop of the cable atop one of the utility poles, which you can see in the photo above. (The loop is highlighted in red.) “If the fiber is in a linear shape, a sound interacts with the fiber just once and then keeps traveling,” says Ozharar. “But if you have a coil, the same signal travels multiple times through the fiber.” That makes the system much more sensitive, like recording a concert with multiple microphones, instead of one fan in the crowd bootlegging it with their smartphone.

When Brood X emerged in the spring of 2021, Ozharar’s DAS system was accidentally listening in. This kind of “periodical cicada” develops underground and emerges every 13 or 17 years to mate, depending on the species. “Because of perhaps climate change—although we’re not exactly sure the reason—there have been stragglers, so populations that have come out early and populations that have come out later than what they’re metabolically timed to do,” says Ware. “Having a way to over time monitor those can be really helpful.”

Male cicadas have an organ, called the tymbal, that vibrates like a drum to produce that unmistakable song. Each species has its own variation on the song, allowing the right males and females to find each other. There’s extra information embedded in that sound, too: Males tend to call during the hottest time of day, which is energetically expensive. That allows females to assess the quality of their mates—they want to choose the fittest males so they can pass primo genes to their offspring.

Hence all the noise. DAS can listen from the very beginning of the emergence through the peak and into the decline as the mass mating ritual wanes. The volume of noise is a solid indicator of the number of cicadas, so entomologists can work out the population size of the brood. They can even see the effect of temperature: When it’s hotter, it’s more difficult for the male cicadas to sing. “You can see that as you go across the five days from which we have monitoring data, that when it’s slightly colder temperatures they have slightly different frequencies in hertz of the calling,” says Ware.

Fiber optic cables are already all over the place, just waiting for scientists to tap into them. They are abundant in cities, of course, but they also run between them, which would be handy for entomologists who want to monitor insects in more rural areas. “We use them just to transmit the data—zeros and ones—but we can do much more,” says Ozharar. “That’s why fiber sensing will become more and more important, and more widely used, in the near future.”

Not that anyone’s suggesting DAS will replace other ways of monitoring insects—fiber optics are widespread, but they’re not everywhere. Instead, DAS could complement other techniques. A field called bioacoustics already uses microphones to listen for species in remote areas, sometimes assisted by AI to parse the data. This method could help confirm the data coming from the fiber optics. Scientists are also experimenting with “environmental DNA,” or eDNA, for instance using air quality stations to gather the biological material floating in a given area. And entomologists like Ware still need to collect specimens from the field to physically examine the health of individual animals.

“What seems really cool about this new technology is that you have this single cable that can cover potentially many kilometers, and all of the information is getting recorded by a single device,” says Elliott Smeds, an entomologist and research associate at the California Academy of Sciences, who wasn’t involved in the research. “Especially now that insects are declining, we’re realizing that we don’t even know what the baseline is for a lot of these species, to keep track of how they’re doing. The biggest obstacle is having enough boots on the ground to be collecting this kind of data.”

The trick will be adapting DAS to monitor species that aren’t the loudest insects on Earth. “In this case, it was very clear these were cicadas, because there were—without exaggeration—millions of them that suddenly descended,” says Ware. “But in most cases, the populations are much smaller for each species. Knowing whether or not we can actually distinguish among insects will be an interesting question.”

This story originally appeared on wired.com.