The explosive gas known as "fire damp" that has stalled the rescue of 29 men at a New Zealand colliery is a miner's worst nightmare, a former worker at the mine said Saturday.
The miner, who asked not to be named, said he and colleagues at the remote pike river mine were warned about the dangers of the gas, commonly called methane, at inductions before commencing work on the project.
"It was also going to be a problem there because it's hard coking coal, which has high levels of methane," he told AFP.
The miner, who worked on the mine's construction and left before it began production in 2009, said a fault line ran through the area where the colliery is located, resulting in a number of cave ins during tunnelling.
He said while the mine operated to high safety standards, the combination of methane-heavy coal and seismic activity made it a difficult environment to work in.
There were further dangers from carbon monoxide, a poisonous gas with no odour that is released into confined underground spaces by mining machinery, the miner said.
But he said it was fire damp —the damp comes from the German word for vapour "dampf" — that was the chief fear of all coal miners.
"The fire damp's like your worst nightmare, it can just go off. This is the last thing you want to happen when you're working underground," he said.
The Pike River mine uses a 2.5 kilometre (1.5 mile) tunnel running beneath the Paparoa mountains to reach the Brunner coal seam, which was discovered in 1864 and is the foundation of the west coast's coal mining industry.
Attempts to tap the rich seam have led to at least two disasters in the past.
In 1896, an explosion killed 65 miners at the brunner mine in New Zealand's worst mining disaster, then in 1968 another blast killed 19 people at the Strongman mine, which was sited just across the paparoa range from pike river.
Mining expert Bruce Hebblewhite, who heads the engineering school at the Australia's University of New South wales, said rescuers at the mine would be concerned about a follow-up gas explosion.
"That's why, as i understand, they are taking a long time to make sure that they can assess that the environment is stable," he told AFP.
He said one of the problems with methane, which is always present in underground coal mines, was that it was difficult to detect, making it hard to know when it was reaching an explosive level.
While it is not known what triggered the pike river mine blast, Mr Hebblewhite said miners worked hard to ensure there were no sources of ignition underground.
"There are some extreme situations where you can get what is known as a frictional ignition where a lump of rock strikes another or strikes a metal and generates a spark, but that's pretty uncommon," he said.
He said the situation was serious. "The consequences of gas explosions can be quite extreme," he said.