A series of minor tremors recorded on the Danish Baltic island of Bornholm has puzzled scientists, who now say they were caused by “acoustic pressure waves from an unknown source”.

At first the tremors on Saturday were thought to have been caused by earthquakes. Then, seismologists theorised they originated from controlled explosions in Poland, more than 90 miles (140km) to the south.

On Monday, the Geological Survey of Denmark and Greenland (Geus), an official body that monitors the underground, said the tremors were “no