Rescuers got hope to find six trapped coal miners - images of an intact chamber with potentially breathable air and mysterious sounds in the mountain.
Officials were studying the results of air quality samples taken from a third borehole, and were expected to release their findings Thursday, 10 days after the cave-in.
Video images from the same shaft showed an undamaged section complete with a ventilation curtain that divides intake air from exhaust air. Behind the curtain, in theory, the men might have found refuge and breathable air.
As daylight broke on the mountain Thursday, there was no indication when company officials would begin drilling a fourth hole. The hole had been planned for more than a day, but its location was changed after noise was detected Wednesday by devices monitoring vibrations in the mountain, raising some hope the men might be found alive.
"If the men went in there, they could be alive," mine co-owner Bob Murray told The Associated Press late Wednesday. "There was no damage at all. The roof is intact; no ribs have outburst. The floors are in place - it looked just as it did when we mined it."
The sounds detected could be a rock breaking underground or even an animal, said Mine Health and Safety Administration chief Richard Stickler.
"We saw some indication of noise for a period of about five minutes that we had not seen before," Stickler said. The source of the noise was not known.
Plans for the location of a fourth borehole had changed because of the "unusual" noise readings.
As crews slowly dig a path to the men's presumed location at the Crandall Canyon Mine, the choice of where to drill the narrow holes sunk deep into the mountain amount to little more than educated guesses.
"There are a lot of possibilities," Stickler said. "We started with logical thinking: 'If I were in this situation, what would I do?' That has guided us in where we look."
The men, if they survived the Aug. 6 collapse, could be huddled together or spread out anywhere in an underground area the size of several football fields.
"There's always a chance. You have to hang on to that chance. But realistically it is small, quite small," said J. Davitt McAteer, former head of the Mine Health and Safety Administration. "You would have to have every single break and divine intervention to successfully extract these guys."
An air sample taken earlier from another part of the mine showed barely 7 percent oxygen - not enough to support life. But Stickler said the collapse could have forced breathable air deep into the mine near the third bore hole, about 1,300 feet (396 meters) to the west of the initial reading.
Mining rescues after 10 or more days are not unheard of. In May 2006, two miners were rescued after being trapped for 14 days following a collapse at an Australian mine. In 1968, six miners were rescued after 10 days in West Virginia.
The effort to dig out a rubble-filled tunnel was proceeding slowly Wednesday and could last another week to go more than 1,200 feet (366 meters) before reaching the area where the miners were believed to be working.
"We're going to get there - no doubt about it," Bodee Allred, the mine's safety manager, said Wednesday in his coal-blackened overalls.
Allred, who has a cousin trapped inside the mine, said the force of the collapse was "definitely something I've never seen before."
The thunderous collapse blew out the walls of mine shafts, filling them with rubble. If the men were not crushed by rock, their bodies could have been crushed by the immense air pressure generated by the collapse, mining executives and federal regulators have said.
And if they survived that, they could have died from lack of oxygen, even though fresh air is now being pumped down one of the drill holes.