The Helsinki University Institute of Seismology said on Tuesday that Finnish seismologists discovered five undersea explosions in Russian seas of the Baltic Sea last week.

According to the Richter scale, which is used to gauge the strength of seismic activity, “we have recorded five explosions, the largest of them having been of 1.8 in magnitude and the smallest of them being of 1.3,” the institute’s director Timo Tiira told Reuters.

Four explosions were recorded on Thursday and one on Friday in Russian territorial waters in the Gulf of Finland, Tiira said, adding it had been clear that the observations had been caused by blasts and not by other seismic activity.

“It can be seen in the shape of the signal and in the content of its frequency,” he said.

The institute did not know what had caused the blasts but that similar disturbances had been detected during previous naval drills and sweeping of old mines from the sea bottom, Tiira said.

“We have detected explosions in that same area before, but not very often, not even every year,” he said.

The Russian Ministry of Defence was not immediately available for comment.