Spent the day doing some surveying in a gypsum mine...they needed alignment for and overhead conveyor that had washed out from flooding. We were working in a side tunnel not far below the surface....not somewhere I was expecting to see a D8 go by. It pretty much filled up the tunnel and could only turn around at intersections. Kind of interesting to see, but I'm not sure I'd care to work down there all the time.
Years ago I worked in the local mines. Operated d4, d5 and d6 dozers underground at various times. They were used mostly for roadbuilding or pushing the ore out of the stopes, levelling sandfill etc.