Nine people were trapped underground following gas explosion Thursday in a colliery in north China's Hebei Province, local work safety authorities said. The accident occurred at about 1 p.m. at Dadi Coal Mine in Chengde County of Chengde City, according to the provincial work safety administration. The mine's production licence expired in June 2006, and was suspended from production for rectification, the administration said. The mine has an annual production capacity of 40,000 tons. Rescue work had been launched. Investigation into the cause of the accident is underway. 