Hundreds of rescuers are continuing to search for 22 miners who became trapped underground after a gas leak occurred in a coal mine in southwest China\'s Yunnan Province Thursday morning. Ventilation machines are pumping excess methane gas out of the mine, while rescuers are taking turns braving high levels of gas, coal dust and debris in the pit to find survivors in the Sizhuang Coal Mine, located near the city of Qujing.\"We must rescue the trapped miners at all costs,\" said Zhao Tiechui, deputy director of the State Administration of Work Safety, at the site early Friday. Coal dust forced out of the shaft by expanding gas has blanketed the area around the shaft\'s entrance. The area has been cordoned off by police and 30 ambulances are parked outside the mine. Local work safety officials said the private mine was operating without a license when the accident occurred. The mine\'s license was revoked a year ago. \"It is an accident that should not have happened. It exposes a lack of supervision by the local government. We must learn a lesson from this accident,\" Zhao said. A preliminary investigation showed that the gas leak occurred at one platform inside the shaft but later spread to another platform, trapping a total of 43 miners on the two platforms at about 6:30 a.m. Thursday. Twenty-one of the miners have already perished. \"The rescue has been difficult because the gas levels remain high and may lead to an explosion at any time,\" Tan Xiaopeng, a fire control official in charge of the rescue effort, said late Thursday.