Rescuers are working around the clock in their efforts to reach six Utah coal miners who remain trapped more than 450 metres below ground, but their initial effort is being called a failure.

"I'm very disappointed. That's one step backward," Robert Murray, chairman of Murray Energy Corp. of Cleveland, part owner of the Crandall Canyon mine, said at a Monday evening briefing.

They still haven't made contact with the six in the Crandall Canyon Mine, who were trapped by a early Monday cave-in so powerful it initially got confused with an earthquake.

In fact, seismologists registered shockwaves measuring a 4.0 on the Richter scale. Authorities say the rumbling was likely caused by the mine collapse, but couldn't conclusively rule out an earthquake.

Despite the devastation, owners of the mine say the crew has an excellent chance of surviving.

"There is plenty of air for them to survive for days," Murray said. He also noted the mines are equipped with drinking water.

The miners have a minimal risk of being exposed to carbon monoxide as there wasn't a fire or explosion. However, they would still have to survive the collapse of the roof.

They are believed to be about 6.5 kilometres from the mine's entrance. At one point, the rescuers thought they were about 500 metres from the trapped men.

"The idea is to get a hole into where they are," Murray said.

"They could be in a chamber 1,000 feet long or they could be dead. We just don't know right now."

The mine uses a method called "retreat mining," which utilizes pillars of coal to hold up the mine's roof.

When the area is completely mined, the company removes the pillar and causes an intentional collapse. Experts say that is a very dangerous technique.

"The whole problem has been caused by an earthquake,'' Murray angrily insisted.

Government inspectors have issued 325 citations against the mine since January 2004, the Associated Press reported. Of those, 116 were called "significant and substantial."

One expert told AP that the mine's record is in the medium range.

"I believe we run a very safe coal mine. We've had an excellent record," Murray said.

Utah had 13 underground coal mines in 2005, the most recent year for which statistics are available. In Canada, underground mines only operate in B.C. and Alberta.

Coal mining in Canada is a reasonably safe industrial occupation, claims the Coal Association of Canada. Three underground miners have died in the past six years.

"It's still too many," Allen Wright, the association's executive director told Â鶹ӰÊÓ.

"You've got a better chance of getting hurt working in a department store than you will in a mine. Does that mean it doesn't happen or that it can't happen? It can happen and periodically it does."

With a report from CTV's Sarah Galashan and files from The Associated Press