The second explosion "vindicated" the decision not to send rescue teams in to find 29 trapped miners including two Britons in New Zealand, a mining expert said today.
Police said the workers would not have survived the "horrific" second explosion at the Pike River mine in Atarau on the country's South Island and rescue teams were "now in recovery mode".
Pete Rodger, 40, from Perthshire, and Malcolm Campbell, 25, from St Andrews, Fife, are among the men missing following Friday's initial blast.
Rescue teams were unable to go into the mine after the first explosion because of high levels of toxic gases.
Patrick Foster, a senior lecturer in mining engineering at the Camborne School of Mines, part of the University of Exeter, said if rescuers had been sent in and the explosion had taken place it would have been "far worse".
He said: "We won't know what triggered off the second explosion until the investigation is completed. It's probably safe to assume either the methane levels built up or they were high and got diluted, then something ignited it.
"Methane is explosive when there is a mixture of 5% to 15% methane in air.
"An existing fire underground could have ignited it. It needs a spark to ignite it or some sort of ignition source.
"Positively the decision not to send rescuers in has been vindicated. If they had gone in it would have been far worse.
"Carbon monoxide is hazardous even in quite low concentrations. If the miners were not able to get to a safe haven where there would be oxygen then they would probably have been overcome by carbon monoxide."
Dr Foster worked as a mine safety consultant for five years in the UK and overseas including South Africa and India before becoming a lecturer. He has been lecturing at the University of Exeter for 10 years.
Speaking about what caused the explosions, he said: "I think you have to assume it's methane. There are other possibilities.
"One of the known hazards is when there is an explosion in a coal mine it can lift the coal dust from the floor and the flame front that follows it could ignite the coal dust causing a secondary explosion. However, you have to assume methane played a contributing role.
"The methane is always there. It can be very low percentages or very high percentages in the coal. It will seep out into the air from the coal seams."
Dr Foster, who has a PhD in mine safety, continued: "All we can do as mining engineers is control that and minimise that and provide ventilation systems that dilute methane so that it is negligible.
"There are sensors and barriers underground and other control measures used in mines.
"In the UK alarms sensors go off when there is one and a quarter - 1.25% - the equipment shuts off automatically and when it reaches 2.5% you withdraw the persons.
"The main investigation question will be what caused that high level of methane in the mine.
"These events in the 21st century are thankfully very rare. However, methane has probably killed more miners in the history of mining than anything else."Reuse content