abandoned
catacomb
cave
cavern
coal
concrete
corridor
danger
dangerous
dark
deep
dig
dirt
dirty
dungeon
entrance
equipment
exploration
geology
industrial
industry
inside
interior
iron
lines
lining
metal
mine
mineral
mining
mystery
obsolete
old
ore
passage
prospecting
rail
railroad
railway
remnant
rock
rusty
scrap
shaft
tunnel
tunneling
underground
underpass
urban