Seven coal miners are missing after a tremor struck a mine in southern Poland, a spokeswoman for a coal mining company said Saturday.

A rescue operation was launched at 11:25 am Saturday after a tremor occurred at the Zofiowka coal mine, said Katarzyna Jablonska-Bajer of the Jastrzebie Coal Company. The mine is located in the southern town of Jastrzebie-Zdroj near Poland’s border with the Czech Republic.

She said four miners were brought to the surface but contact was lost with seven others who were 900 meters (2,950 feet) underground, preparing a new corridor for extraction works.

“There has never been such a powerful tremor at the mine,” she said.

The State Mining Authority said the tremor had a magnitude of 3.4 on the Richter scale. TVN24 said the tremor was also felt on the surface and shook some houses.

At first, high methane levels in the mine prevented rescuers from trying to reach the site, but after four hours they began to head there, Jablonska-Bajer said.

Coal mining is a major industry in Poland and coal remains the main source of energy and heating, but Poland is taking some steps to shift toward renewable, cleaner sources of energy.

The Main Statistical Office says some 65.8 million metric tons (58.7 million tonnes) of coal were extracted last year, some 4.8 million tons less than in 2016.