Mysterious reports of tremors on a small Danish island in the Baltic Sea prompted a seismological investigation that points toward atmospheric pressure waves as the possible culprit. But as with other, similar events—which are surprisingly common around the world—confirming a cause remains elusive.

The tremors occurred on May 13 at around 3 P.M. local time, according to a statement from the Geological Survey of Denmark and Greenland, which operates two seismographs on the island of Bornholm.

The seismic data showed that the tremors were not caused by an earthquake, the agency says. And although the tremors occurred about 20 minutes after what might have been a minor blast in Poland, the scientists don’t think that caused the strange shaking, either.

Instead, agency officials concluded that “acoustic pressure waves” in the atmosphere likely triggered the tremors—but they declined to speculate about what might have been responsible for those pressure waves. Acoustic waves, or sound waves, carry energy from a vibrating object and cause the pressure of the material they pass through (such as air) to oscillate.

Such unexplained tremors actually aren’t unusual, says Björn Lund, an earthquake scientist at Uppsala University in Sweden and director of the Swedish National Seismic Network. The network received about 10 reports of tremors from the part of southern Sweden just north of the island on the same afternoon and evening.

“This happens a couple of times a year,” Lund says. “We have reports of shaking and low-frequency noise from a very large area, and we have no indications in the seismic data of earthquakes or major blasts.” One common explanation for this sort of incident is an airplane breaking the sound barrier, he says, although he added that he hasn’t heard of any such activity occurring around these specific reports.

(Sweden does record about 1,000 earthquakes per year, Lund says—on average, two or three each day—but most are much too small for people to feel, much less report.)

Strange tremor reports with no clear geological explanation aren’t limited to Scandinavia, says Allison Bent, a seismologist at Natural Resources Canada. Bent says her office regularly receives calls from people who have felt the ground shake—but when scientists check the seismographs, there’s not always the signature of a genuine earthquake or even a sign of other geological activity, such as a mining blast or heavy construction.

“Most of the time, we can’t tell them what it was,” Bent says. “We can offer suggestions because we know what things tend to look and feel like earthquakes.” In Canada, she says, these imposters can also include so-called frost quakes, which occur when very wet soil rapidly freezes.

“They all feel the same when they’re at the small end of things,” Bent says of these assorted events. That makes deciphering seismological signals more difficult, she notes. “As long as the ground shakes, we will record it,” she says, but “unless it’s something large and coherent, we usually don’t pick it up, or don’t pick it up as an identifiable signal.”

Nevertheless, seismometers—which is technically the internal mechanism of a seismograph, although the two terms are often used interchangeably—are incredibly powerful instruments that can help scientists understand many aspects of Earth beyond geological activity, says Wendy Bohon, an earthquake geologist and science communicator.

“Seismometers record anything that causes the Earth to vibrate, and that can be lots of different things,” Bohon says. That includes earthquakes, of course—but also human activity ranging from urban traffic to bomb detonations, as well as the constant hum of Earth’s oceans, atmospheric phenomena such as hurricanes and even meteors slamming into Earth’s atmosphere. And then there are “fan quakes,” the vibrations seismometers can detect from energetically loud fans at a stadium.

“These things are happening all the time,” Bohon says of strange shakes such as those reported over the weekend on Bornholm. “It’s not, like, unusual—it’s just that as our technology gets better and better, and we’re able to record more and more subtle vibrations, we learn more and more about how all of the things on Earth are connected.”