Published: Nov 23, 2010 12:02 AM EST
Updated: Nov 22, 2010 8:59 PM EST

GREYMOUTH, New Zealand (AP) - The bid to rescue 29 New Zealand coal miners trapped underground by a massive gas explosion ran into more problems Tuesday as a mechanical robot broke down inside a
tunnel and hard rock layers slowed progress on drilling to test the air.

Police superintendent Gary Knowles said the army robot sent in to transmit pictures and assess toxic gas levels was damaged by water and out of commission. Authorities were urgently seeking other such robots from West Australia and the United States to replace the broken one, Knowles said.

"I won't send people in to recover a robot if their lives are in danger," he said. "Toxicity is still too unstable to send rescue teams in."

Making matters worse, the drilling team boring into the mine tunnel had hit "very hard rock" overnight, Knowles said. The police superintendent's statements came as rescuers waited impatiently for a chance to test if air quality underground was safe enough for them to go in to pull out the miners, who have been trapped for nearly five days.

Family members have expressed frustration with the pace of the response as officials acknowledge it may be too late to save the miners, who have not been heard from since a massive explosion ripped through the Pike River Mine on the country's South Island on Friday.

A buildup of methane gas is the suspected cause of the explosion. And now the presence of that gas and others - some of them believed to be coming from a smoldering fire deep underground - are delaying a rescue over fears they could still explode.

A diamond-tipped drill was put to work as workers hit layers of hard rock and came within 33 feet (10 meters) of the tunnel where they believe some of the miners are trapped, police superintendent Gary Knowles said. The 500-foot (160-meter)-long shaft they are creating will allow them to sample gas levels - including explosive methane and carbon dioxide - and determine if rescuers can finally move in days after the blast.

Knowles said rescuers planned to drop a listening device down the hole to see if they could hear anything - such as tapping sounds - that might indicate that the miners were still alive.

"This is a very serious situation and the longer it goes on, hopes fade, and we have to be realistic. We will not go underground until the environment is safe," Knowles said.

Two workers stumbled out of the mine within hours of Friday's explosion, but there has been no contact at all with the remaining 29. A phone line deep inside the mine has rung unanswered.

"The families are showing grief, frustration and probably anger," said Laurie Drew, whose 21-year-old son, Zen, is among the missing. "I have my moments I can keep it together but deep down my heart's bleeding like everybody else's." Knowles appealed for patience.

"You can't put men underground as a rescue team until it's a safe environment," he said. "I've looked at other rescuers and the chance of rescue teams dying or being critically injured (in a fresh explosion) is great."

Those trapped include a teenager who was so excited about his new job he persuaded mine bosses to let him start his first shift three days early - on the day of the deadly gas explosion - his mother told local media.

Joseph Dunbar was one day past his 17th birthday and the youngest among them when he joined his fellow miners in the pit.

Mine shift supervisor Gary Campbell said Dunbar was desperate to be part of the team.

His mother, Philippa Timms, said her son "got offered this chance to have a career - and that's how he saw it, as a career," she told TV One.

The wait to begin the rescue bid for the men had been frustrating, but Timms said she understood why.

"They can't just rush in there because, I know right from the word go, I know how it works," she said. "If the oxygen rushes in and it hits that methane, then bam, they're gone, (in) another blast."

Police have said the miners, aged 17 to 62, are believed to be about 1.2 miles (two kilometers) down the tunnel.

Each miner carried 30 minutes of oxygen, and more fresh air was stored in the mine, along with food and water that could allow them to survive for several days, officials say.

New Zealand's mines are generally safe. A total of 181 people have been killed in the country's mines in 114 years. The worst disaster was in March 1896, when 65 died in a gas explosion. Friday's explosion occurred in the same coal seam.

--- Lilley reported from Wellington, New Zealand.

(Copyright 2010 by The Associated Press. All Rights Reserved.)