Six trapped coal miners failed to respond to rescuers pounding on a second drill that has pierced a collapsed mine, and water pouring through the drill hole foiled a first attempt to use a camera to see the space, rescue officials said.
Rescuers next plan to insert a lining into the new drill hole, which will take many hours but would protect the camera and let it send back good images, they said.
The miners have not been heard from since Monday, when part of the Crandall Canyon Mine near Huntington, Utah, collapsed.
There is potable water and a "survivable space" where the second, nearly 9-inch (20-cm), drill punched through the mine ceiling, US Mine Safety and Health Administration chief Richard Stickler said at a news conference.
Rescuers hope the reinserted camera will be able to scan 100 feet (30 metres) in each direction and provide better details of the conditions underground.
A first, 2-1/2-inch (6-cm) drill pierced into another area of the mine late on Thursday but there was no sign of life when a two-way microphone was lowered into the mine.
Rescuers said survey equipment later showed the first drill had "drifted" some 85 feet (26 metres) as it made its way down from the surface and may not have pierced the cavity where the men were thought to be stranded.
Tests taken with instruments fed through the first hole showed that the oxygen levels in the chamber were too low to sustain life, although no other toxic gases were detected.