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Updated 5:51 PM- HUNTINGTON -- Rescue workers who drilled into the Crandall Canyon mine to in hopes of locating six miners trapped 1,700 feet underground are keeping their spirits up despite discouraging developments last night.

First, rescuers were elated after finally drilling a 2 1/2-inch hole through to the Emery County mine. But when a microphone was lowered down the bore hole, it detected no sounds that might indicate the miners had survived Monday's mine collapse.

And early this morning, rescuers were perplexed to learn that the high oxygen levels they first found had dropped.

"I can't explain," said Robert Stickler, assistant secretary of the federal Mine Safety and Health Administration (MSHA).

One possibility, he said, is that the drill bit may have "drifted" on its way down and punched into an abandoned section of the mine.

But if the drill had reached the cavity where mine operators believe the miners to be, an air sensor found too little oxygen to support life, officials said.

"The thing to do is to continue on our plan and maintain our hope," Stickler said, adding that the miners' families, briefed prior to the press conference, took the news well.

"While we are disappointed ... their spirits are still up. They still have hope," he said. "We all should continue to have hope."

Stickler said the six readings of oxygen dropped in a matter of hours from about 20.5 percent, which would sustain life, to a meager 7.5 percent, which would not.

He could not explain the drop, but acknowledged the early sample may have been flawed when the drill was maneuvered.

The drillers immediately lowered a survey device that should confirm if the bore hole reached the area where the miners are believed to be. But geological variations along the drill's route could have diverted the drill into an abandoned and sealed part of the mine, Stickler explained.

Two other factors also are in play. Stickler said some readings from air samples indicated carbon monoxide levels of 180 parts per million, which could be deadly over long periods of exposure.

Jim Holms, a physician at LDS Hospital in Salt Lake City, said time and exposure levels are key to determining carbon monoxide's impact on a person. Brief exposures to high levels can be deadly, but lower levels can also be detrimental.

The 180 parts per million "is not extremely high, but long exposures at long levels can cause problems," said Holms, who is part of the Intermountain Hyperbaric Medical Center.

The Occupational Safety and Health Administration limits workplace exposure to carbon monoxide to 50 ppm for up to 8 hours, Holms noted. Longer exposure to higher levels can lead to heart and brain problems, he said.

The potential for hypothermia also is worrisome. Mine officials have said the mine temperature is a constant 58 degrees - not a dangerous temperature on its own, said Colin Grissom, a critical care physician and wilderness medical expert at LDS Hospital.

But if the miners' clothing is wet, they could suffer from hypothermia, or low body temperature, he said.

Being wet in cool conditions can cause a 25 percent increase in body heat loss, Grissom said.

"If they stayed dry, there's every reason to think that wouldn't be a factor," he said.

In other developments, a 8 5/8-inch drill that is boring into the mine from another direction was expected to reach the mine this afternoon. The larger bore hole would allow larger, more sophisticated cameras to look as far as 100 feet into the cavity for signs of life.

And Stickler said that workers digging through the mine from its entrance have advanced 400 feet inside.

"Those effort continue around the clock with a great deal of enthusiasm and hope and faith," Stickler said. "The workers' attitude is tremendous."