Explosive and poisonous gases had prevented rescuers from entering the mine to search for the missing men since an initial explosion on Friday at the Pike River Mine. But even if any had survived, the second blast would have killed them, police superintendent Gary Knowles said.
"Unfortunately I have to inform the public of New Zealand at 2:37 p.m. today there was another massive explosion underground and based on that explosion no one would have survived," said Knowles, in charge of the rescue operation. "The blast was prolific, just as severe as the first blast."
The second blast came after the first progress in days for the rescue attempt, when a drilling team broke a narrow shaft through to the section of the mine where the 29 workers were believed to have been working. And two robots had crawled their way into the tunnel, giving authorities their first view of the inside of the mine.
It was not immediately known what triggered the second blast, which came almost exactly five days after the initial explosion. Pike River chief executive said the rescue teams were not doing anything that could have triggered it.
"It was a natural eventuation, it could have happened on the second day, it could have happened on the third day," he told reporters.
In recent days, officials became increasingly pessimistic about the chances of pulling the men alive from a network of tunnels some 1 1/2 miles (2 kilometers) deep in the side of a mountain, following the first powerful explosion on Friday. Nothing had been heard from the missing miners since that blast.
Earlier Wednesday, drillers using a diamond-tipped drill bit to prevent sparks finished boring a 530-ft. (162-meter) hole to the mine's main tunnel. It was a key step, giving officials their first information from that section of the mine and allowing testing for levels of dangerous gases.
Hot air and gas rushed the hole when the chamber roof was punctured, and Whittall said earlier Wednesday that initial tests showed it was "extremely high in carbon monoxide, very high in methane and fairly low in oxygen." Carbon monoxide - the polluting gas from car exhausts - is extremely poisonous, while explosive methane is the gas believed to have ignited in Friday's blast.
And, an army robot usually used for bomb disposal had crawled two-thirds of a mile (1 kilometer) into the tunnel on Wednesday and found a miner's helmet with its fixed light still glowing.
Officials said the helmet belonged to one of two miners who managed to escape the initial blast.
Security footage of Friday's blast shows a wall of white dust surging from the mine entrance and small stones rolling past for about 50 seconds as the force of the blast rips out of the mine. The dust was blown across a valley and the blast wave shot up a ventilation shaft, tearing off surface vents hundreds of feet above.
New Zealand's mines have been safe historically, with 181 deaths in 114 years. The worst disaster was in 1896, when 65 died in a gas explosion. Friday's explosion occurred in the same coal seam.
Lilley reported from Wellington, New Zealand.