Emergency services are conducting a "difficult rescue operation" after four miners were trapped by water in the Gleision Colliery in Pontardawe, Swansea.
Fifty rescuers are currently involved in the operation, including specially trained firefighters, rope specialists and experts in urban search and rescue. Yet despite their efforts, the trapped men are facing the prospect of a night underground.
The miners, who have been named as Philip Hill, 45, Charles Bresnan, 62, Garry Jenkins, 39, and David Powell, 50, are believed to be stuck in a ventilation shaft. The colliery is a drift mine, offering only horizontal access for the rescuers.
The alarm was raised at around 9.20am. A retaining wall holding back a body of water underground failed, flooding a tunnel that the seven men were in.
Chris Margetts of South Wales Fire Service said he was “very hopeful and optimistic” that four miners who are trapped 90 metres underground can be rescued.
He added that the miners were located around 250 metres into the shaft. The route out of the mine is blocked by water, which the emergency services are trying to pump out.
"Conditions are favourable," said Margetts. "Air supply is good."
When asked how the accident happened, he said: "The reason the mine flooded is because the wall to an old working failed. This is a body of water that went from where it was being held into an area where it should not be, that cut off their exit."
There has been no contact between the surface and the trapped men, but rescuers believe the four, all fit and healthy seasoned miners, would have fled to an air pocket to await rescue.
Speaking at the site, shadow Welsh secretary Peter Hain said that he was satisfied that everything was being done, though he conceded that it "is a grim situation".
Prime Minister David Cameron said on Thursday that “every support” would be given to emergency services trying to rescue the miners, adding that his “thoughts are with those missing and their family and friends”.
Speaking earlier on Thursday, Superintendent Phil Davies of South Wales Police, confirmed that seven men had originally been trapped. Three had escaped. One had been taken to Morriston Hospital in Swansea.
"A rescue operation is under way," he said. "As you can imagine, it is quite a dynamic situation."
Local councillor Arthur Threlfall told the BBC: "The mine is in quite a remote spot. At the moment you cannot go anywhere near it because a large area around it has been cordoned off by the police."
"This is the first mining disaster I have known for many years. There are not many collieries left like there used to be."
An emergency centre has been set up within the community hall in the nearby village of Rhos to cater for the families of the miners.