On Saturday, May 13, over 60 people contacted the Geological Survey of Denmark and Greenland (GEUS) to report tremors on the Danish Baltic island of Bornholm.
Residents described a deep rumbling, shaking, and change of pressure in their ears when the tremors hit, AP News reports, while the GEUS recorded tremors with a magnitude of 2.3. At first, the cause was assumed to be an earthquake, but that has been ruled out by seismologists at the GEUS.
Instead, they believe the tremors were caused by ”acoustic pressure waves from an unknown source". There was another suspect for the unexplained quakes; military exercises carried out in Poland, over 140 kilometers (87 miles) to the South. However, that, along with earthquakes, was ruled out.
“The seismologists can report that it is unlikely that the tremors originate from a controlled explosion in Poland, which was carried out shortly before the first reports of tremors on Bornholm,” GEUS said in a statement seen by AP News.
The tremors did not cause any injuries, though they did reportedly crack the walls of a house. Though the GEUS is baffled as to the source of the pressure waves, there are a few possibilities.
"What usually happens when we receive reports of rumbling and shaking is almost always that the air force has passed through the sound barrier at sea, and the atmospheric conditions have been such that the sound bang rolled in over land," Björn Lund, Associate Professor of Earth Sciences at Uppsala University, told Swedish newspaper Dagens Nyheter.
Lund added that under certain weather conditions, the sonic boom can seem to come in over the land.
"What you feel is not actually shaking in the ground, but the pressure wave in the air," he clarified.
Similar reports of earthquakes came in from residents of the northwest of England in 2022, which were later determined to be caused by a Typhoon combat aircraft breaking the sound barrier off the coast. For now, though, the exact cause of the tremors felt at Bornholm remains unknown.