The Australian mining expert involved in the recovery work at the Pike River coal mine says the West Coast mine is the most challenging he has encountered.
Dr David Cliff of the University of Queensland says he has never been to a mine that presents so many natural difficulties.
He says capping the shaft could be done by crane in Australia.
But at Pike River, it will have to be done by helicopter and helicopters cannot hover over a fire or a thermal draft.
A GAG unit, a converted jet engine brought from Australia, is now operating continuously.
Twenty nine men are dead after a series of explosions at the mine since 19 November.