Efforts to dig out two miners buried for nearly two days beneath at least 45 feet of rocks and dirt in an open pit coal mine resumed Thursday morning after a seven-hour safety delay, a federal official said.

Bob Cornett, acting district manager for the Mine Safety and Health Administration, said excavation was halted overnight so a crew could detonate explosives to bring down material that had threatened to fall into the work site from a 150-foot-high wall in Tri-Star Mining Inc.'s Job No. 3 mine.

"We're in a situation now where we want to be as safe as we can," Cornett said.

He said the miners' chances of survival diminished with each passing hour.

The two were trapped Tuesday morning when the bottom of the high wall collapsed, burying them and the equipment they were operating, Cornett said. Trucks have been hauling about 2,500 tons of debris each hour from the pile at the western Maryland mine since then to try to reach them.

The collapse created a layer of rocks ranging from 45 feet to 100 feet deep, but rescuers believe the collapse pushed the men and their equipment toward the shallower end.

"There are some very large rocks on that side that you can see gaps, spaces, vacuums or holes that potentially, if the machinery was pushed that way, there could be air pockets," Cornett said.

The miners were working at the bottom of the pit when the wall collapsed. One was operating a tracked backhoe and the other was using a loader. Both have CB radios, although the men have not communicated with anyone since the collapse.

Cornett said he did not believe the machines were equipped with emergency breathing apparatus. The so-called "self-contained self-rescuers" provide an hour of oxygen.

The cause of the collapse was not known, but Cornett said that rain tends to cause more high wall failures at this time of year.

"When you have rain like we've been having here, and freezing and thawing, it may have had an impact," he said.

The site is in a rugged area of mountains called Georges Creek and is near the Eastern Continental Divide. The region is dotted with abandoned mines and fading former company towns.

The mine has had no fatal injuries since at least 1995 and was not cited for violations in its most recent inspection, which began March 5, according to MSHA. The mine employed 51 people at the end of 2006 and produced nearly 653,000 tons of coal last year.

Owner George R. Beener was at the site, according to a woman who answered the phone at a mine office. She said the company had no comment and declined to identify herself.

Tri-Star Mining Inc. had 15 to 20 people working on the rescue. Joining them were a crew of eight from MSHA and local firefighters.