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Grieving family members leave after a Sunday briefing from Pike River Coal management on the 29 miners and contractors trapped underground at the Pike River mine, Greymouth, New Zealand. ((Ross Setford, NZPA/Associated Press))

Rescuers prepared to drill a gas-sampling hole through 150 metres of hard rock and into a New Zealand coal mine Sunday where a powerful blast trapped 29 workers nearly two days ago.

Pike River Mine Ltd. chief executive Peter Whittall told reporters a small 15-centimetre-wide hole would be drilled into the mine from the mountain above over the next 16 to 24 hours to enable rescuers to sample gas levels from deep in the mine's centre.

Underground combustion that continues to generate dangerous gases was preventing rescuers from entering the mine.

"We've got a heating of some sort underground and that means there's some combustion generating the gases that go with that, carbon monoxide, a slight increase in methane and some other gases," Whitehall said. "Something is happening underground, but what it is we don't know."

The small 15-centimetre hole, to be drilled through the mountainside, was aimed at an area a couple of hundred metres further into the mine from where the main ventilation shaft is located.

"That will give us more information about what's going on" with gas levels near the centre of the mine, he said.

Not safe for rescuers

Dangerous methane and carbon monoxide gas levels meant "it's still not safe for rescue teams to enter the mine," Whittall said, though "the oxygen quality coming out of the mine is still high."

Fresh air is still being pumped into the mine through an open air line.

Police search controller, Supt. Gary Knowles, said the "primary focus today [Sunday] is still a rescue operation.

"Our major focus is bringing these guys home," he told reporters, "but we need a stable air environment. When we have a stable environment … and as soon as we have that we're going in."

Anguished relatives voiced frustration over the delays in the rescue operation.

"If I had my way I'd be down there, I'd go into the mine myself," said Laurie Drew, whose 21-year-old son, Zen, is one of the missing men.

Police said the miners, aged 17 to 62, are believed to be about two kilometres down the main tunnel.

Two men emerged after the explosion Friday, but there has been no word from the 29 others.

Electricity in the mine went out shortly before the explosion and that failure may have caused ventilation problems and contributed to a buildup of gas.

Whittall noted the blast was most likely caused by coal gas igniting.

The power outage continued to frustrate efforts Saturday to pump in fresh air and make it safe for rescuers, though Whittall said air was flowing freely through a compressed air line damaged in the explosion.

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Pike River Mine Ltd.'s chief executive Peter Whittall speaks Saturday at a media conference on the rescue of 29 workers missing after an explosion at the Pike River mine in Greymouth, New Zealand. (( Ross Setford, NZPA/Associated Press))

"We have kept those compressors going and we are pumping fresh air into the mine somewhere. It is quite conceivable there is a large number of men sitting around the end of that open pipe waiting and wondering why we are taking our time getting to them," Whittall said.

A working phone line to the bottom of the mine, however, had rung unanswered.

The two dazed and slightly injured miners stumbled to the surface hours after the blast shot up the mine's 108-metre-long ventilation shaft. The men were taken to a hospital for treatment of minor injuries and were being interviewed to determine what happened. Whittall said one of the two men had used the phone to contact the surface before walking out.

The explosion occurred about 3:45 p.m. Friday. Video from the scene showed blackened trees and light smoke billowing from the top of the rugged mountain where the mine is located, near Atarau on South Island. It is New Zealand's largest underground coal mine.

Families of the missing men gathered at a Red Cross hall in nearby Greymouth on Saturday, and were being briefed hourly on rescue efforts. Most have declined to talk to reporters, as have the two men who made it out of the mine. They are expected to be taken to the mine site on Sunday.

Knowles said the families "are pretty distraught … but they understand the risks we're facing."

PM visits families

After visiting the families, Prime Minister John Key told reporters that "there is a great sense of anxiety and genuine fear, and I think that's only natural given the … difficulty of the situation."

"We reflected to them that they have to hang on to hope. As we saw in the case of the Chilean mine, 33 miners did get out alive," he said.

But unlike the accident in Chile, where 33 men were rescued from a gold and copper mine after being trapped one kilometre underground for 69 days, Pike River officials have to worry about the presence of methane, mine safety expert David Feickert said.

He added, however, that the Pike River mine has two exits, while the mine in Chile had only one access shaft that was blocked.

The coal seam at the mine is reached through a 2.3-kilometre horizontal tunnel into the mountain. The seam lies about 200 metres beneath the surface. According to the company's website, the vertical ventilation shaft rises 108 metres from the tunnel to the surface.

Whittall said the horizontal tunnel would make any rescue easier than a steep-angled shaft.