Cu
abysm
abyss
alveolation
alveolus
antrum
aperture
armpit
arroyo
atrium
basin
booth
bowl
box
box canyon
breach
break
broaching
bug
canyon
cell
cellule
chamber
chap
chasm
check
chimney
chink
cistern
clearing
cleft
cleuch
clough
col
compartment
concave
concavity
coulee
couloir
crack
cranny
crater
crevasse
crevice
crib
crypt
cup
cut
cwm
decay
cave
excavate
excavation
excavator
artery
canal
cavern
depression
fissure
gland
hole
membrane
mound
muscle
opening
organ
orifice
pit
ridge
structure
tissue
tube
tunnel
vault
depravity
gravity
projection
protuberance