crew arrived ten minutes later. High levels of carbon monoxide (CO) and methane gas in the mine atmosphere made it necessary for rescuers to wait 12 hours after the explosion to begin to reach the miners. Tests taken through holes drilled from the surface showed that the air near where the miners were last known to be stationed contained 1,300 parts per million of CO. More than 200 parts per million is considered unsafe. However, each miner had a Self-Contained Self-Rescuer (SCSR) device that provided one hour of breathable air. Emergency supplies were stored in 55-gallon drums (205 L drums) within the mine. Even after the gases abated, rescue teams had to proceed with caution, continually testing for hazards such as water seeps, explosive gas concentrations, and unsafe roof conditions. This limited their rate of progress to an hour. They checked in every , and then disconnected their telephones until the next checkpoint in order to avoid the possibility of a spark creating another explosion. MSHA had deployed a 1,300-lb. (520 kg) robot into the mine as well, but pulled it out after it became mired from the mine entrance. After more than nine hours of searching, rescue teams pulled out of the mine at about 3:40 a.m. Tuesday, January 3. Through an agency spokeswoman, Bob Friend of MSHA said the teams were withdrawn when they discovered that the mine's atmospheric monitoring system was still running. Due to the air quality in the mine, power to the system could have caused another explosion, according to safety experts. A borehole being drilled to check the mine's air quality was nearing the mine roof. "The bit and steel being used was not equipped to use water, which meant the bit was hot and could ignite an explosive mixture of methane," Friend told a reporter from the West Virginia Gazette. Rescue teams returned to the mine 6:22 a.m. Locating the trapped miners The 13 trapped miners were about inside the mine at approximately below ground. Five four-man teams tried to make their way through the entries, which were high. By 12:40 p.m. on January 3, they had reached into the mine. It was believed that the trapped miners were somewhere between from the entrance. Two holes were drilled from the surface into areas where the miners were believed to be; microphones and video cameras lowered into them for ten-minute periods did not find