abandoned
adit
cave
cavern
coal
concrete
corridor
danger
dangerous
dark
decay
deep
derelict
digger
dirt
dirty
dungeon
exploration
industrial
industry
inside
iron
limestone
lining
metal
metro
mine
mine exploration
mineral
mining
mystery
old
ore
passage
prospecting
rail
railroad
railway
rock
rust
rusty
shaft
subterranean
trolley
tunnel
underground
underpass
urban
water