It is described as a vast subterranean network of interconnected caverns and tunnels, stretching beneath entire continents and forming an underworld for surface settings.
[5] It was also part of the Eberron campaign setting, in which it was called Khyber and was home to evil beings driven deep into the caverns at the end of the age of demons.
[8] Much of the literary attention for this deep underworld stemmed from the sourcebooks and accessories for the Forgotten Realms setting, including R.A. Salvatore's novels about the fictional character Drizzt Do'Urden.
[8] The fictional Underdark's physical characteristics are based upon conditions in real-world caverns deep underground, except at immense size.
In the Forgotten Realms setting, Araumycos (Dwarvish, literally meaning "Great Fungus") is an enormous fungal growth in the Upper Underdark under the continent of Faerûn.
It is a single organism living beneath the High Forest between one and three miles under the surface, immune to magic and resistant to psionic energy.