Q&A: Why has a rescue not started for trapped NZ miners?

Your questions answered about the delay in rescuing 29 New Zealand coal miners trapped underground since an explosion ion Friday.

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Rescue teams have still not been given the all-clear to search for the 29 men trapped in a New Zealand coal mine, almost three days after a suspected methane gas explosion ripped through the colliery.

Friends and family on the surface are becoming increasingly frustrated with the slow pace of rescue efforts.

Why has a rescue not started?
Authorities have not allowed rescuers to enter the mine because of the presence of methane gas, which is believed to have caused the initial explosion. Methane, a common gas encountered in coal mines such as the Pike River colliery, is toxic in large quantities, odourless and highly flammable.

Rescuers have been held back for fear an accidental spark triggered by entering the mine could cause another explosion.

Methane levels are being constantly monitored for signs they are dropping, something which has not yet been noted.

Why can't rescuers enter with breathing apparatus?
They could, but toxicity of gases inside the mine is not the main problem. The real fear is that the mine is a powder keg of gases, which could explode again at any time. Authorities already suspect coal is smouldering inside the mine after the initial explosion, which means another blast is possible.

Professor Bruce Hebblewhite, head of mining and engineering at the University of New South Wales, said: "What they are most worried about is ignition from any existing combustion.

"They will have self-contained breathing apparatus which is all good for breathing in an irrespirable atmosphere, but it's no good against an explosion."

Is the mine blocked?
Based on what authorities know, and reports from two men who walked out of the mine after the explosion, the main shaft is not believed to be blocked. Some machinery is reported to be in the middle of the 5 metre-wide shaft, which is dug horizontally into the side of a mountain range, but rescuers would have a gap of about 1 metre to manoeuvre around the machinery.

What is known about the mine?
The Pike River mine is on the rugged and sparsely populated west coast of the South Island, an area rich in coal and with a long mining history. Pike River is close to the sites of New Zealand's two worst mining disasters, and its most recent.

The Pike River mine is driven into the side of the Paparoa mountain range. It consists of an access shaft about 2.5 kilometres long, which slopes upward and is never more than about 150 metres from the surface.

At the end of the shaft is a small network of tunnels to work the coal seam. This area is relatively small as the access tunnel only broke through to the coal in April 2010.

The mine has two vertical ventilation shafts, which along with a compressed air system help circulate air around the mine.

What is known about the explosion?
At about 3.45pm local time on Friday a massive explosion occurred at a point about 1.5km along the main shaft. The explosion caused a fireball which went up one of the ventilation shafts and scorched an area of surrounding bush.

There were 31 men in the mine at the time, including two who walked to safety out of the main entrance immediately after the explosion. The location and fate of the others is unknown.

Tests have indicated there is heat coming from within the mine, which could be a smouldering fire.

Are the miners likely to be alive?
Authorities have from the start refused to speculate on how likely it is that the men are still alive.

It is not known how far along the shaft the explosion travelled, and whether methane is present near the rock face where most, if not all the men, are believed to be.

An emergency phone near the coal face has been rung constantly with no answer.

It is believed the air near the coal face remains fresh, and the miners could be in a safe zone awaiting rescue. The miners have no food but there is natural water seepage into the mine.