From ABC News:
A series of minor tremors recorded on the Danish Baltic island of Bornholm Saturday has puzzled scientists, who now say they were caused by ”acoustic pressure waves from an unknown source.”
At first the tremors were thought to have been caused by earthquakes. Then, seismologists theorized that they originated from controlled explosions in Poland, more than 140 kilometers (nearly 90 miles) to the south. On Monday, the Geological Survey of Denmark and Greenland, an official body that monitors the underground, said the tremors were “not caused by earthquakes, but by pressure waves from an event in the atmosphere.” However, they came from “an unknown source.”