Underground

*Natural caves
*Fountains (springs), grottos
*Hewn caves
**Tunnels
**Mines
**Dwarf caverns
**Delvings
*Cellars, Cistern, Cesspit
*Undercrofts
*Crypts
*Catacombs
*Barrows, dolmens, cairns
*Vaults
https://en.wikipedia.org/wiki/City_of_Caves
https://en.wikipedia.org/wiki/Dungeon
https://www.academia.edu/11000129/Castles_as_Prisons
Category:Topography
Category:Geography
Category:Geology
Category:Structures