Crews moved thousands of tons of rock Wednesday in a bid to find two men trapped under at least 40 feet of rubble at a surface coal mine.

Part of one high wall of the open pit mine collapsed Tuesday, burying the men as they operated machinery, said Bob Cornett, acting district manager for the federal Mine Safety and Health Administration.

The wall at the Tri-Star Job No. 3 mine near Barton was 100 to 125 feet high. Its collapse created a layer of rocks that ranged from 40 feet to 80 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.

Workers used a large power shovel to load trucks with rock at a rate of about 2,500 tons an hour Wednesday, but work was slowed by the instability of the debris and a boulder the size of two pickup trucks, Cornett said.

He said one man was using a tracked backhoe and one was using a loader. Both pieces of equipment have enclosed cabs and CB radios, although the miners had not communicated with anyone since the collapse.

"We will continue this as a rescue operation until we know it's not," he said.

The cause of the collapse was not known, but the heavy rain during the weekend could have been a factor, Cornett said.

The miners' families were being given hourly updates.

The mine in western Maryland is operated by Tri-Star Mining Inc. of Barton. 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 would not identify herself.

According to MSHA, 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. It employed 51 people at the end of 2006 and produced nearly 653,000 tons of coal last year.

The company operates at least two other surface mines in Maryland, according to Ron Wyatt, a family liaison for MSHA.