STOCKHOLM (AP) — An earthquake struck a northern Swedish mine on Monday, with officials saying it was one of the strongest ever measured in the country. While 13 people were in the mine at the time, no one was injured.

The Swedish National Seismic Network said the earthquake was magnitude 4.1, just shy of the 4.3 earthquake measured in southern Sweden in December 2008.

The shake occurred shortly after 3 a.m. in the town of Kiruna. The mine is said to be the largest and most modern underground iron ore mine in the world. The Norwegian Seismic Array NORSAR said the earthquake was tied to the ore extraction activities.

Fredrik Bjorkenwall, a spokesman for the company that owns the mine, LKAB, told Sweden’s Aftonbladet tabloid that the water level in the Kiruna mine was rising but pumps were working. He said there were 13 people in the mine at the time of the shake, but no one was injured.