Rescue teams have renewed their efforts to dig out four trapped miners in a West Virginia mine, two days after an explosion killed at least 25 of their co-workers. It is the worst U.S. mine disaster in a quarter century.

 

Crews used giant drills to dig deep into the earth to determine if it's safe for rescue workers to go in. Officials at Massey Energy, the owner of the mine, say the quality of air must be tested.

Rescuers were drilling four boreholes more than 300 metres deep in a bid to vent the mine and hit a long chamber where the missing miners may have sought refuge.

Mine officials say the drilling could take another several hours.