Hope survives despite the odds. As the whole country waits with the families of the 29 men in the Pike River mine, it clings to the words of rescue co-ordinators that the miners are "trapped" and the recovery operation remains "in rescue mode".

But hope is attended by dismay that a mine explosion could still happen under modern standards of coal extraction and air monitoring.

"We have risk management. Safety management plans, we have ventilation management plans. We have first-response management plans," said Pike River chief executive Peter Whittall yesterday. "This is a unique event."

It is not clear what makes it unique. Methane and carbon monoxide have been risks for as long as coal has been mined. It has been mined on the West Coast since the 19th century.

The Brunner mine disaster of 1896 was 20km away from Pike River. All 65 of those underground lost their lives when coal dust ignited. At the Strongman mine 19 died when a wrongly fired charge sparked a methane gas explosion in 1967.

Despite centuries of development, coal mining remains prone to gas explosions, yet it evidently has not developed rescue technology that can go quickly into an airless and potentially explosive environment when there is a possibility that lives could be saved.

Rescue services have been poised at the portal of the Pike River mine since Friday afternoon, held back by the risk that their movements could cause another explosion.

They are beginning a fifth day of waiting for air sampling to say when it may be safe to enter the mine. The strain on the families and the frustration of everyone concerned have been mounting with each passing hour. Information has been scarce.

The authorities have been unable to say what is happening within the dark and silent tunnel except that air conditions are not stable.

"The analysis we have at the moment suggests something else is occurring," said the general manager of New Zealand Mines Rescue yesterday, "but there is limited information."

We have learned from the account the Herald obtained from survivor Daniel Rockhouse, published yesterday, that a compressed air line gave him oxygen to replace the carbon monoxide filling his lungs and enabled him to haul himself and a fellow miner to safety.

If that line was still functioning after the explosion on Friday afternoon, it might still be. It is a thread of hope.

So is the narrow hole being drilled down 162m to a mine shaft yesterday so that better air tests might be taken and pictures provided by laser imaging equipment and video cameras. Any data these devices can provide about conditions below ground would be better than the information vacuum prevailing for the past three and a half days.

Rescuers are being held back on suspicion the conditions pose too great a hazard; if they are erring on the side of caution there is still hope.

History does not support much optimism. The Brunner and Strongman disasters have more recent echoes around the world. An American reporter recalled on NewstalkZB yesterday that rescuers spent four days drilling bore holes in an attempt to release gas from a mine in West Virginia where an explosion claimed the lives of 29 miners.

But he gave a reminder that this year's Chilean rescue, albeit not a combustible coal mine, showed the power of hope. "For 17 days they didn't know if anyone was alive," he said. "Great things can happen."

The eyes of the world are on the remote mine in the New Zealand bush and the thoughts of many are never far from the families of the missing and the efforts that are waiting to be made to reach their loved ones.

Their patience has been stoic. May their hope be rewarded.