This might have been part of the Pioneer Pellet Plant. It looks to be a ball mill, which pulverizes ore by spinning it with thousands of ball bearings.
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...based on the tags: flywheels, graffiti, mills, pillars, pipes, rusty...
The left cave is the largest of the three, and shows the most evidence of expansion.
The middle section of the smokestacks were coal hoppers, and this device would load the coal into the hoppers from the conveyor belt it rode across. The bottom section of the stacks were storage rooms while the very top were, surprise, chimneys for the power plant.
A panoramic view of the sintering plant’s gas plant (?). Everyone who visits must get a picture of these rusty smokestacks!
I found a historical photo of this room showing 10-foot high machines with wires hanging by the mile from looms and schematic charts.
Taken at a junction in the tube world.
Look at the floor–do you see the hole? That goes down a lonnnnnng ways.
Judging by the bed, this room was used by employees in its later years.
One of the machines left over in the underground magnetic separation plant.
A collapsing and unstable building.