Hmmm Urban tunnel explorers

[NorthernRaider]

I always wanted to get into the old anhydrite mines up here, they are 600 ft down and huge both in extensivness and size of tunnnels, so big in fact ICI used to strip 32 ton trucks down and reassemble for use down the mine. Then of course there is the mega sized Boulby Porrash mines that are even bigger.