Divers have been sent to a flooded mine as efforts continue to rescue four miners trapped 90m (295ft) underground.
The men have been trapped by water at Gleision Colliery near Cilybebyll, Pontardawe in the Swansea Valley since Thursday morning.
They have been named as Charles Bresnan, 62; David Powell, 50; and Garry Jenkins, 39, from the Swansea Valley; and Phillip Hill, 45, of Neath.
Two others managed to escape while a third is in a critical condition.
Rescuers have made no contact with them but are optimistic they will be freed.
Prime Minister David Cameron, who is returning from a trip to Libya, said his thoughts were with the men and their families "at this very difficult time".
Labour leader Ed Miliband said the country was "watching, waiting and hoping for a successful rescue".
Neath MP Peter Hain said that emergency services were using divers to establish conditions in the drift mine tunnel - close to the river Tawe.
"I wouldn't want to underestimate the significance and seriousness and grimness of the plight of these miners," he said.
"They are in a desperate situation. Rescuing them is going to be very difficult. Pumping out the water is going on all night."
Rescuers have brought in a company from Cheshire which has produced a device which can detect sound underground.
Philip Shaw, of Sure Wave Technology, said: "As long as the men are making a noise we will be able to say definitely that they are alive. We don't even need to be in the mine."
He said all miners were trained to make a noise in these sorts of situations.
Chris Margetts, from Mid and West Wales Fire and Rescue Service, said water was being pumped from the mine and oxygen was being pumped in.
"What we have determined is the miners are located approximately 90m underground," he said.
"They are down a 250m main route into the mine... there are numerous little tunnels and old workings which all potentially have air pockets in.
"They are experienced miners, they know the layout of the mine, they would know where to go in this situation.
"What we are dealing with is their egress out of the mine is full of water."
He said conditions in the mine were favourable and he was "very hopeful and optimistic" the men could be freed successfully.
BBC Wales News website reporter Daniel Davies, who is at the scene, said relatives of the miners - gathered at the Rhos Community Centre meeting point where they can talk to police and specialists - had been preparing for a long night.
"At about 19:45, the Red Cross delivered bedding to the community centre where the relatives are, such as pillows and blankets," he said.
He added that assembly member Bethan Jenkins had told him how local people had been bringing food and drink supplies.
Ali Thomas, leader of Neath Port Talbot Council, described the miners as "great guys" and said he had spoken to some of their families.
"I know at least two of the four and I'm quietly optimistic, given that they might have found a safe haven this morning, that they will still be there," he said.
It is understood two of the miners involved are father and son, with the son escaping earlier while the father was still trapped.
The men became trapped by water after breaking into old mine workings, it is believed.
'Working very hard'
Wayne Thomas, from the National Union of Mineworkers in south Wales, said the best equipment and the best expertise was combining to help the men.
"These men have worked in the mining industry for most of their lives and they will have experienced the challenges of mining and the difficulty that brings," he said. "And they will know people are coming towards them. It's only a matter of time."
Despite reports that the source of the water may have come from an underground reservoir, Andrew Watson, who runs the Mines Rescue Service and is co-ordinating the rescue, said the source was not yet established.
"The most essential bit of kit we've got is long-duration breathing apparatus and access to pumps which will remove the water out of the mine," he said.
Mine owners MNS said their thoughts were with the families.
First Minister Carwyn Jones said the mine was underneath a steep slope near the River Tawe.
"Nevertheless, we know the emergency services are working very hard and we know the mine is in a difficult position to reach," he said.
Retired mine director Keith Jones, 69, from nearby Alltwen, who used to own a mine on the same side of the mountain as Gleision Colliery, said conditions would be "stone cold freezing".
Meanwhile, a dedicated telephone line has been set up for members of the public to contact if they are concerned that relatives may have been involved. The number is 01792 555565.
Emergency services, called to the mine at 09:21 BST, said it was not yet known exactly how the incident happened.
Two men escaped unaided from the drift mine - a mine cut into the side of a hill where the coal seam is accessed horizontally - before help arrived.
Supt Phil Davies said though there was water in the mine there was nothing else preventing access.
Phil White, a former director of Tower Colliery, in Rhondda Cynon Taf, which closed in 2008, said: "Gleision Colliery has been working for some time under a private licence.
"It has been a long-established mine. It's not a big mine and it would employ 10 to 20 people."
Some 50 rescuers have been involved in the operation, of whom 18 to 20 were firefighters trained for this type of incident.
Rescuers included nine rope specialists and 12 urban search and rescue specialists.
Welsh Secretary Cheryl Gillan commented: "I am receiving regular updates from the police services and I will be praying for the safe rescue of all involved."
Although most mines in south Wales are now closed, there are pockets of small-scale collieries still in operation.
Gleision Colliery, in operation since 1993, works coal under a very steep hillside above the banks of the river Tawe.