Rescuers in the US state of Utah have drilled through to a mine cavity where six miners are believed trapped, but have not yet detected any sound.
The rescue crew drilled a narrow hole and lowered a microphone into the collapsed Crandall Canyon coal mine.
No contact has been made with the miners, who have been trapped 1,500ft (457m) underground since Monday.
Tests carried out at the site showed the air quality was good, the mine's co-owner told reporters.
"That means if they're alive, they're going to stay alive in that atmosphere," said Bob Murray, chairman of Murray Energy Corporation.
CRANDALL CANYON MINE
Located in Manti-La Sal National Forest, 16 miles (25km) from Huntington
Uses the "retreat mining" method, which experts say is one of most dangerous
Coal pillars hold up area of mine's roof. When completely mined, pillars are pulled, causing intentional roof collapse
He said a 2.5-inch (6.4cm) drill had pierced through the cavity on Thursday night.
But asked if the miners were definitely in the chamber, Mr Murray said: "No, we can't be sure but we think so. We think that's where they are... They didn't move far, I'm sure, from that area".
Rescuers are still working on drilling a wider 9-inch (23cm) hole through which a camera could be passed to check for signs of life, and water and food could be delivered.
Mr Murray said it could take at least another four or five days to bring the men out.
Rescue teams have been drilling round-the-clock through a mountain above the mine to reach the men, who include three Mexican citizens.
The group is thought to be 3.4 miles (5.5km) from the mine entrance, 140 miles (225km) from Salt Lake City.
Initially the collapse was blamed on an earthquake, but experts have since suggested the shafts might have caved in with enough force to register on seismographs.
This has led to speculation mining procedures could have triggered the incident.
The drilling is still continuing on a wider hole
It has been claimed the workers were using a risky technique known as "retreat mining", where the last standing pillars of coal are pulled down and the roof is allowed to fall in.
But Mr Murray has rejected this, saying an earthquake caused the collapse.
US federal mining inspectors have issued 325 citations for alleged safety violations at the mine since January 2004. Of those, 116 were considered "significant and substantial" and likely to cause injury.
Experts have said the number of citations is not unusual, and the mine's owners say they run a safe mine.