NZ mine explosion: Test shaft nearing completion
Rescuers are close to completing the drilling of a borehole into a mine in New Zealand where 29 men have been missing since an explosion on Friday.
The 15cm (6in) wide shaft had reached a depth of 142m (466ft) by 2230 (0930 GMT), 20m short of the area where the miners are believed to be trapped.
Probes will be lowered down it to check for gas and listen for signs of life. A robot will also be sent into the mine.
Prime Minister John Key says there is "every chance" the men are still alive.
However, senior officials also acknowledge that they are planning for all eventualities, including the "possible loss of life".
There has been no contact with the miners - 24 New Zealanders, two Australians, two Britons and a South African - since Friday afternoon.
They were working at the Pike River mine, near Greymouth on South Island, when there was a large explosion, believed to have been caused by methane gas.
Dangerous levels of methane and carbon monoxide inside the mine have hampered efforts by rescue crews to reach them.
On Monday evening, the mine's operator, Pike River Coal, said the drilling of the test shaft was progressing well, but that it had slowed after a band of harder rock was reached.
Earlier, chief executive Peter Whittall said a special diamond drill bit would be used for the last 10m (33ft), to minimise the risk of sparks which could set off another devastating explosion.
"Once we do [break through], the collective wisdom on site is that the air will want to come up the borehole rather than down it so we'd expect that we'll start getting gas samples through that hole fairly quickly."
"Then we can start deploying a number of different techniques down the hole," he added.
As well as taking gas samples, laser imaging equipment and a listening device will be lowered down the borehole to assess the situation.
''It will create a proper image of what's down there," Mr Whittall said.
He added that a New Zealand Army robot was being prepared to send into the mine. The remotely operated vehicle will be able to travel up 2.5km (1.5 miles) and is capable of recording images.
It will be used first to check the area where an underground loader is thought to be blocking the access way. One of the two men who have so far emerged from the mine since the explosion was operating the loader.
However, David Cliff, an associate professor at the minerals industry safety and health centre at the University of Queensland in Australia, warned that using robots in similar situations in the past had seldom worked.
"They are not able to cope with the level of damage found in the mines and are not flame-proof or intrinsically safe," he said. "They usually trail a cable for control and communications and these are prone to damage."
A fibre-optic cable was also being laid from the mine's entrance to the top of the main ventilation shaft, officials said. It should allow rescuers to monitor any changes in the air coming out of the mine.
Police Supt Gary Knowles, who is in charge of the rescue operation, told reporters that he remained optimistic.
"We are still keeping an open mind, but we are planning for all outcomes and... also under this process we are planning for the possible loss of life as a result of what has occurred underground," he said.
But it was still too dangerous for a rescue team to enter the mine, because gas samples from the main ventilation shaft were still showing high levels of carbon dioxide and methane, and the possibility of a fire underground, he added.
"We need to establish beyond reasonable doubt that an emission source does not exist. For this reason we in the process are establishing another sample point through another bore hole."
"I am going to reinforce the fact that we are doing this to ensure the safety of those miners underground and also the teams that have to go and rescue them."
Mr Key said it was possible that the miners might have found refuge in a ventilated area and could still be rescued.
"The advice I have is that there is oxygen in the mine and there is every chance that those miners have managed to get to a pocket of that oxygen flow and therefore that they are alive," he said.
Each miner carried 30 minutes of oxygen, enough to reach oxygen stores in the mine that would allow them to survive for several days. Fresh air is also being pumped into the mine through a shaft, although it is likely to be hot and stuffy.
While the men would reportedly have been carrying flasks of water, there is no food underground. Their cap lamps will run out after about 24 hours.
The prime minister's determination will give comfort to the families and friends of the missing men, says the BBC's Phil Mercer in Greymouth.
For three days they have waited for the slightest sign that the men are alive, but there has been only silence, our correspondent adds.
The mayor of Greymouth, Tony Kokshoorn, said locals felt as if they were "in limbo" as they waited to discover the miners' fate.
"Every hour and every day that goes by we have to face the facts that they can't stay down there indefinitely," he said.
"We've got to get some resolution, some movement forward, because the desperation is turning to grief."
Tearful family members have been taken on tours of the mine site to help them understand the dangers of an attempted rescue, and to show them the procedures being practised in preparation for an eventual operation.
Laurie Drew, whose 21-year-old son Zen is among those trapped, said it was hard seeing a board with the men's name tags missing.
"We have got faith that they are going to come out... but it's just how long is it going to take to find out for sure what's going on down there," he said. "We have just got to be strong for our loved ones and hope they come out."
Two workers who walked out of the mine on Friday have been discharged from hospital after receiving treatment for moderate injuries.