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The Pike River Recovery Agency says the team working underground encountered an unusual substance on the walls and roof of the tunnel and took samples.
Last week, a loader driven by one of the two men to survive the Pike River Mine explosion 10 years ago was pulled out of the drift by the Pike River Recovery Agency underground team.
It was being driven by Russell Smith on November 19 in 2010 when the mine exploded, killing 29 men.
The recovery team reached the machine at the 1575-metre mark, and removed it by pushing and pulling it with loaders on either side.
"The fifth and last robot was recovered before the loader, and now there's several hundred metres of previously unexplored drift and roadways in the Pit Bottom in Stone area that have not been seen since 2010.
"We've reached a significant point in the recovery which is about trying to find out why the 29 men who went to work on 19 November, 2010 didn't come home."
The $51 million recovery is nearing the end of the drift (entry tunnel) and the project.