WELLINGTON, New Zealand – Rescuers today prepared to drill a gas sampling hole through 500 feet of hard rock and into a New Zealand coal mine where a powerful blast trapped 29 workers nearly two days ago.

Pike River Mine Ltd. chief executive Peter Whittall told reporters a small 6-inch-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 center.

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,” Whittall said. “Something is happening underground, but what it is we don’t know.”

The 6-inch hole was aimed at an area 660 feet farther 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 center of the mine, he said.

Dangerous methane and carbon monoxide gas levels meant “it’s still not safe for rescue teams to enter the mine,” Whittall said, although “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 superintendent Gary Knowles said the “primary focus today is still a rescue operation.”

“Our major focus is bringing these guys home,” he said, “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 1.2 miles 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, although Whittall said air was flowing freely through a compressed air line damaged in the explosion.