Eight days since the collapse of a Utah coal mine trapped six miners underground, ongoing rescue efforts failed Monday to detect any signs of the men, or whether any of them were still alive.

Crews were preparing to drill a third and fouth hole in hopes that the miners may have fled to other areas of the mine. Two bore holes have reached what mining officials have described as "survivable" space, but have not revealed any signs of the miners.

The drilling locations are being chosen based on calculations of where the miners, trained in survival protocols, would have gone to escape the collapse, Richard Stickler, head of the Mine Safety and Health Administration, said.

Crews are using the holes to pump air into the mine, and are lowering video cameras into the holes in search of signs of life.

Concerns for the safety of rescue workers has frustrated efforts to reach the miners underground, Stickler said. The collapse forced large amounts of oxygen deficient air into the mine and officials must determine the source of the air and the direction in which it is flowing.

"We need to know this information ahead of time for the safety of rescue workers who may encounter oxygen deficient air," Stickler said.

"I am very disappointed on the eighth day to not have found six alive miners," Bob Murray, head of Murray Energy Corp. and co-onwer of the Crandall Canyon mine said at a press conference Monday.

Murray was planning on heading underground at 4 a.m. Tuesday morning to photograph and video tape the underground rescue efforts. He said he planned to show the footage to the miners' families and the media Tuesday.

"These are the worst mining conditions I have ever seen in my 50 years of mining," Murray said of the underground conditions slowing efforts to dig through to the miners. "We must not risk the lives of the rescue teams who are all my employees," he said.

However, the discovery by video cameras that the mine roof was in tact, of survivable air space and plenty of potable water underground were all reasons to be optimistic, Murray said.

"I want to emphasize that there are many reasons to have hope, many, many reasons to believe the miners are alive," Murray said.

An attempt to sink a video camera deep inside the coal mine yielded no signs of life, officials said Monday.

The camera was paired with better lighting, but still only saw about 15 feet when it was lowered into the mine overnight, said Al Davis, who oversees Western operations for the MSHA. The images that came back included a distorted conveyor belt and an intact roof.

A video camera lowered into the collapsed mine Sunday revealed equipment, but no sign of the miners.

"I've accepted all possibilites," said an exhausted Cody Allred, the 32-year-old son of missing miner Kerry Allred.

One of the four miners who escaped alive said Sunday he didn't feel or hear a thing as the mountain shook and caved in, trapping six of his colleagues.

Tim Curtis was near the mine's entrance on Aug. 6 when he got a text message telling him of the collapse on his PED, or personal emergency device. The trapped men are believed to be about 3.4 miles from the mine's entrance.

"Where I was at, I felt nothing," Curtis said in an interview with The Associated Press. "It's just like you are here and three miles away are you going to hear a balloon pop?"

Curtis, a 33-year-old third-generation miner who works as a mine fire boss, or safety inspector, has worked 12-hour shifts every day since the collapse to aid the rescue effort.

Two holes have already been drilled about 1,800 feet down into the mountain containing the mine. The first is a 2 1/2-inch wide hole that rescue crews initially believed drifted during the drilling process into a neighboring sealed chamber. A microphone dropped into that first hole heard no sign of the miners and air samples recorded an atmosphere of only 7 percent oxygen — measurements similar to those known to exist in a sealed area of the mine and an oxygen level that would not sustain life.

The hole was later determined to be in an active work area, and rescuers were pumping air down the hole.

A second hole measuring nearly 9 inches allowed crews to lower a camera into a cavern that officials said showed a "survivable space." But images were limited and the camera was withdrawn to clear off one lens.

The second attempt to use the camera to get a glimpse of the missing men was hampered by poor lighting that limited the camera view to only about 15 feet into a 5 1/2-foot-high void at the bottom of the hole, far less than the 100 feet it's capable of viewing, said Stickler.

Rescuers saw a tool bag, a chain and other items that are normally seen underground in a mine, he said. "We did not see any sign at all of any of the miners," Stickler said.

Searchers were set to begin drilling the third hole midday Monday, Davis said.

Despite the setbacks, rescue leaders said they were proceeding under the assumption the miners remain alive.

Curtis said the miners likely improvised an emergency plan when all four entry tunnels near their work site were blocked.

"It's just like your house, if it caught on fire and you have a plan to get out the window and that window is blocked," Curtis said. "That's what happened here."

Rescuers have been moving horizontally through the mine to try to reach the men. Their route was blocked about 2,000 feet from the men and rubble had been cleared from about 580 feet of the route, Stickler said.

"We're very distraught," Murray said. "It's heartbreaking that we haven't found them alive. We have spared nothing," he said.