Rescuers managed to drill a third hole into the coal mine in Utah, United States and lowered surveillance equipment that picked up sounds, the owner of Crandall Canyon mine said.

Almost ten days have passed since six miners were trapped underground after a mine tunnel caved in for still unknown reasons. Search operations have been progressing slowly due to seismic activity and large quantity of rock and soil that have to be moved in order to reach the workers.

After two holes drilled from the mountain top didn’t reveal anything that could raise hopes, Robert Murray the mine’s co-owner said sounds were recorded by two geophones from underground.

The five-minute-long sounds gave some dim hope for the rescuers and families, but officials warned they could be made by rock movement or other elements within the mountain.

A video camera was also lowered in the mine chamber and footage revealed an empty space, without any traces of miners. But the intact structure gave hopes that the miners could have survived and fled to another section of the mine. Work at a fourth hole started on Thursday.

The six workers are believed to be trapped at a depth of 500 metres and 6.5 kilometers from the entrance, 134 being divided in teams that work around the clock to reach the chambers left intact by the accident that occurred last Monday.

While the mine’s owner believes the cave-in was caused by an earthquake registered by equipment, specialists said the ground movement recorded by seismographs was caused by the shaft’s crumbling, not a quake.