caves dens grottos tunnels caverns hollows dugouts cavities antres delves grot potholes fissures galleries holes alcoves chambers recess pits excavation depression indentation indentures shafts dint boreholes pockets scoops wells craters concavity dimples dips chasms dents ditch ravines gorges pockmarks channels abysses trenches troughs canyons chutes passages burrows bores tubes sinkholes sags sockets basins niches bottom grooves bowls notches clefts cups crannies gulfs dishes voids curves nooks vacuity cellars basements vaults catacombs crypts