Video from inside Pike River shows a robot that appears to emit smoke or steam when it overheats, but there's no fire or even a flame.
Police have kept the video, which was exclusively obtained by Newshub, from the families of killed miners.
But one relation who's seen it says it shows going into the mine's access tunnel is actually safe, and a mine expert agrees.
The video shows a robot going down the mine's tunnel.
It has to dodge a hanging pipe and it comes across one of its own kind - the wreckage of a capsized robot that failed - before dodging its way past.
It's very wet and the footage shows the robot needs some help, with two men inside too.
Wearing Mines Rescue uniforms and using breathing apparatus, they work casually and without gloves using a pocketknife and duct tape to make a piece of cardboard into a roof.
The robot's journey ends when it gets to an abandoned loader as its way is blocked.
It gets into trouble - overheating - and emitting what appears to be smoke.
Until now, this video has been kept from the families of the 29 men killed, like Dean Dunbar, father of 17-year-old Joe Dunbar.
The footage is six years old, date-stamped March 15, 2011 - three months after the final explosion, and held by police ever since.
"How could they do this to Joseph and the other guys? Why?" Mr Dunbar says.
"They've had the footage; we haven't."
The Government, authorities and the mine's controllers have always maintained it's too dangerous to enter, which Mr Dunbar rejects.
"They created a situation of absolute danger; they made it look and sound explosive. It's not," he says.
"We're going to peel back that litany of lies and expose it for what it is."
The robot was in the mine's access tunnel, known as a "the drift", which was 2.3km long.
The wreckage it goes past is one of two Army robots, abandoned at either 800m or 1050m.
It gets 1570m down the tunnel, to the loader used by Russell Smith - a survivor who got out.
The final 730m are unexplored.
The families say this is crucial, because at 1900m there is an electrical substation that could have evidence, and also because survivor Daniel Rockhouse escaped from about that point as well - meaning there are potentially bodies left behind.
The mine was in police control at the time of the video, but police say it was not passed onto either the families or the Royal Commission because "it was assessed as having no evidential value".
Mr Dunbar says that with no explosion, it proves going into the tunnel is safe and always has been.
"We shouldn't have to convince the good people of New Zealand that your Government has been lying to you for so long, and we shouldn't have to. These guys deserve better, mate," he says.
A spokesperson for Nick Smith, the Acting Minister of Conservation for Pike River issues, says "no one from the Government has viewed this footage".
"Though the Government has been advised it was used by Solid Energy as part of its extensive investigation into whether the mine was safe to enter," they say.
"The fact remains that is unsafe to do so. As we have repeatedly stated, gas levels beyond the seal are consistently around 100 percent methane making a manned re-entry unsafe.
"The Government has, however, agreed to work with Solid Energy and the families to investigate the potential for an unmanned entry. This footage is being reconsidered as part of that investigation, though that work is ongoing.
"The families' technical advisers met last week with Solid Energy, police and WorkSafe to discuss those efforts."