BEIJING (AP) — Rescuers are seeking to free 22 Chinese workers trapped underground for more than two days following an explosion at a partially built gold mine in the country’s east, state media reported Tuesday.

Some 300 rescuers were seeking to clear obstructions to reach the workers, whose condition remains unknown, the reports said.

The mine in the eastern province of Shandong was under construction at the time of the explosion. The cause of the blast has not been announced.

Mine managers did not report the explosion to authorities in the city of Yantai until Monday evening.

China’s mining industry focused on coal used to be the world’s deadliest, suffering more than 5,000 fatalities a year from explosions, floods and gas leaks. Safety improved dramatically after authorities overhauled the industry starting about 15 years ago, boosting supervision and closing most smaller, more dangerous operations.

Accidents still occur relatively frequently, however, including two in the southwestern megacity of Chongqing last year that killed 39 miners.