Terrestrial Environment
Subterranean places comprise one of terrestrial environments, the others being badlands, deserts, forests, hills, marshes, moors, mountains, plains, and swamps.
Subterranean Places of the Realms
Sewers/Cellars: these locations are just beneath the buildings and streets that appear on the surface.
bi-nou, crossroads guardian, crypt spawn, dense pudding, divine minion, ferragan, flareater, gray pudding, hakeashar, Innugaakalikurit, kupuk, greater mimic, phaerimm, rocklord, rockworm, scaladar, scalamagdrion, shadow lurker, silversann, spectator, spectral mage, Statue That Walks, stone pudding, thaalud, thanatar, tirichik. Innugaakalikurit, kupuk, and tirichik are only encountered on the Great Glacier. Statues That Walk are only encountered in Mulhorand.
Dungeons: dungeons are deeper than sewers, but most do not involve delving into the depths of the earth.
bi-nou, crossroads guardian, crypt spawn, dark naga, dense pudding, divine minion, ferragan, flareater, gray pudding, hakeashar, greater mimic, phaerimm, rocklord, rockworm, scaladar, scalamagdrion, shadow lurker, silversann, spectator, spectral mage, Statue That Walks, stone pudding, thaalud, thanatar.
The Underdark: beneath the dungeons is the Underdark; the Realms Below. The Underdark is further divided into the Upperdark, the Middledark, and the Lowerdark.
bi-nou, crossroads guardian, crypt spawn, dark naga, dense pudding, divine minion, ferragan, flareater, gray pudding, hakeashar, greater mimic, phaerimm, rocklord, rockworm, scaladar, scalamagdrion, shadow lurker, silversann, spectator, spectral mage, Statue That Walks, stone pudding, thaalud, thanatar.