The first four hours after Friday's Pike River explosion was the only time rescuers could have entered the mine, a safety expert said yesterday.
After that fleeting period the 2.8km tunnel network became a ticking bomb as it refilled with potentially explosive methane, said former mine general manager John Brady, who has reviewed 20 fatal mine incidents in Australia.
Mr Brady said investigations of mining incidents indicated that a brief window of opportunity arose after an underground gas explosion, when dangerous gases had burned away.
But he stressed that a second explosion often followed immediately. If there were not consecutive explosions, the first four hours was the only period to attempt a rescue or recovery, or repair the ventilation.
Once the methane began to build up again, and without proper air flow, further explosions were inevitable in the following days or weeks.
Some relatives of the 29 miners killed on the West Coast have asked why rescuers did not enter the tunnel immediately after the huge gas-propelled explosion on Friday.
Pike River chief Peter Whittall has stressed that the mine was unpredictable from the moment a wall of flames, smoke and dust blasted 2.2km to the tunnel's entrance and out of a ventilation shaft.
After the blast at 3.50pm, the alarm was not raised for roughly two hours - the time it took for Daniel Rockhouse and Russell Smith to walk 2km out of the carbon monoxide-filled tunnel and alert emergency services.
Mr Brady said if all risk management protocols were followed, rescue teams would never enter a mine after an explosion.
"Unless you were really equipped and ready to go in at that moment, then you can't do it at all. A management team have no alternative but to say, 'We can't send anyone in'."
Every mining expert the Herald spoke to believed that if rescuers had entered the mine on Friday afternoon, it would have been a recovery mission.
The atmosphere in the tunnel was a volatile mix of gases before the second explosion in the Pike River tunnel, preliminary tests showed.
By yesterday morning, drillers had used a diamond-tipped drill bit to bore 162m into the main tunnel without causing sparks. When it broke through, hot air and gas rushed through the hole in the roof where the bore had punctured.
The first tests taken through the borehole confirmed what officials had expected, but hoped not to find: a concentration of 95 per cent methane, high levels of carbon monoxide, and little oxygen.
Carbon monoxide - a colourless, odourless gas - is extremely poisonous, and methane potentially explosive in concentrations of 5 to 15 per cent.
Further samples were taken every 15 minutes and flown to Solid Energy in Rapahoe for analysis. Gas levels will continue to be monitored to determine when the tunnel can be entered.
Mr Whittall emphasised: "We've still got 29 people in there and we need to get them out."
With no cyclical air flow in the tunnel, and methane likely to be seeping from the coal seam, the high gas concentration could take days or weeks to dissipate, hampering efforts to recover the bodies.
The ventilation system was designed to disperse gas, but it is believed to be badly damaged.
Mine explosion and gas analysis expert David Cliff, of the University of Queensland, said four to five hours of samples were needed to understand the stability of the atmosphere.
Rescuers would need to know that the methane levels would stay under 1 per cent.
Engineering and mining geologist David Bell, of the University of Canterbury, said predicting the gas concentrations was difficult. "You're talking days to get a serious trend. It's not going to be resolved in a few hours.
"If you see a downward trend [in gas concentration] over 24 hours, you'll start to get interested. But even then, I understand the first readings were off the scale. It seems it's been spiking quite frequently."
The tactic of pumping fresh air into the tunnel could create an explosive atmosphere, or fuel the flames of any fires that are still burning.
Fleeting period to attempt Pike mine rescue: expert
The ventilation system installed in 2008 to disperse gas. Photo / Greg Bowker
