abri
antre
bunker
burrow
cav
cave
cove
dugout
foxhole
grot
grotto
hole
lair
sewer
subterrane
subterranean
subway
tunnel
warren
core out
enclosure
hollow
hollow out
natural enclosure
caverned
caverning
caverns
alley
canyon
cathedral
cellar
chamber
chasm
cliff
corridor
courtyard
dome
dungeon
fortress
lake
passageway
pit
staircase
tomb
valley
vault
tavern