HUNTINGTON, Utah, Aug 13 (Reuters) - Rescuers prepared to drill a third hole deep into the earth on Monday, the eighth day of what one official termed a "heartbreaking" search for six men trapped in a collapsed Utah coal mine.

Although there has been no sign of the six miners since the Crandall Canyon Mine in central Utah caved in early on Monday, mine co-owner Robert Murray said there were reasons to hope they might still be alive.

"We are all very distraught," Murray told a news conference in the nearby town of Huntington.

"It's heartbreaking, it's absolutely heartbreaking, that we haven't found them alive, but we have spared nothing in the way of equipment and whatever it takes to find these miners as quickly as possible."

Officials say the men could already be dead, crushed more than 1,800 feet (550 metres) below the surface when the mine caved in for reasons that have yet to be determined.

There were reasons to believe they had survived, Murray said, including images from a camera dangled into the mine that showed the roof still intact and the existence of breathable air and potable water.

The images also showed an unidentified miner's tool bag, a conveyor belt and reflectors, but no sign whatsoever of the men themselves.

Rescue crews have twice used drills to pierce into sections of the mine, hoping to contact the missing men, and both times been met with only silence. The third hole is being drilled into an area to where, Murray said, the miners may have retreated after the cave in.

RESCUERS FRIGHTENED

Miners were also burrowing out an escape route big enough for a person, but that effort was going slowly and officials refused to estimate when the tunnel would be complete. Several parts of the rescue operation have taken longer than expected.

Murray, who has insisted that the mine collapse was triggered by an earthquake despite experts' assertions to the contrary, bristled during the news conference at reporters' suggestions that dangerous or improper mining techniques could have been to blame.

But he conceded that about a dozen miners working on the large escape tunnel had been reassigned after expressing fears for their own safety.

"We have had some miners that have been working on the rescue effort that have asked to be relieved from working right at where rescue effort is going forward as they have been somewhat frightened to continue to work there," Murray said.

"If we had any real concerns over safety measures we would not have undertaken this underground rescue," he said.

He said the missing men's families were receiving twice-daily briefings and remained hopeful.

"They're holding up extremely well," Murray said. "Their strength is amazing, and I think it comes from their belief in their God." (Additional reporting by Dan Whitcomb in Los Angeles and Patricia Zengerle in Washington)

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