The journey into the Pike River mine will be an ever-changing experience with constant risk assessment, says one of the men at the centre of the recovery effort.
More than eight years after the first explosion in 2010 that claimed the lives of 29 men, a recovery team yesterday broke through the seal to the mine.
Dinghy Pattinson, the Pike River Recovery Agency chief operating officer and an underground miner of more than four decades, expects it will take the team until the end of the year to go through the entire tunnel.
He has broken down the expected timeline for the recovery process, though stressed the periods of time would depend on unknown conditions inside the tunnel, which they could only assess once they were there.
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What happens now?
"Today what we're doing is we're finishing off some more risk assessment and so forth regarding the work we have to carry out between the 30m wall and the 170m wall," Pattinson said.
The wall 170m into the tunnel was installed in 2011 by a rescue team.
"Tomorrow they will be starting to advance the ventilation."
The team only put enough ventilation into the tunnel yesterday to fill the area they wanted to work in, but that must now be extended to the 170m mark to allow fresh air up to that point.
The next job will be making sure the wall is engineered to be fit for purpose.
"We want to make sure that it maintains the right atmosphere behind the wall," said Pattinson, noting they did not want oxygen getting in behind the wall or nitrogen leaking through.
The process will take a couple of weeks, then the team must wait for about a week to make sure the activity has not disturbed the scene.
"When you start interfering with the underground environment and make changes, you want to make sure it's settled back down."
The wall comes down
The next step will be pulling down the 30m mark wall to allow machinery through into the tunnel.
After that, they will dismantle and remove infrastructure in the space between the two walls.
By the time that is finished, it will have been six to eight weeks since the seal was broken.
"What we still have to do during that time and in that time, we still have to do a lot more planning and risk assessment and evaluations ongoing behind the 170 wall, going right down the end of the tunnel."
Pattinson said there would be a "dynamic risk assessment process" throughout the recovery, because conditions could constantly change.
"There's things we don't know. There will be times as we progress up that tunnel that we will have to stop [and] reassess that plan.
"We don't know what we don't know."
The unknown point
Once the recovery team start moving beyond the 170m wall, progress is likely to be slow.
While it may be faster on some days than others, depending on conditions and what they find, Pattinson said they would probably average a distance of 20-40m a day through the tunnel.
"Definitely the real unknown probably is along past 1400m. There's been robots up to 1400, 1500m, we've got footage of what that is. The real unknowns are behind that."
He couldn't say when they would reach that point, as it depended first on when they actually went through the 170m wall and how fast they progressed.
"I'm picking that it won't take us that long to get up to 1400m.
"I'm still confident that we . . . are still on plan to be at the end of the tunnel, to have a whole lot done by the end of the year."
What could hold up the progress?
There could be safety work that needs to be done as they go further into tunnel, such as re-supporting the roof.
"It's not a fast process," he said.
Forensic work would need to be done each step of the way, as they could not assume that seemingly unremarkable sections of the tunnel did not hold important evidence.
"We've got to treat it all as a crime scene," he said.
"There will some days where you will move quite quickly and there will be other days where you move quite slowly."
Despite the expectations of slow progress and the 16 months it has taken to plan the re-entry, Pattinson said the fact they had finally begun was "exciting".
"[It was] a big achievement yesterday. It's really good for the families.
"We are finally under way."