Underdark
The Underdark is a literal "underworld" to the surface world. It is a vast subterranean network of interconnected caverns and tunnels stretching beneath entire continents.
The Underdark is a strange, alien landscape populated by bizarre creatures and ancient races; nearly all of whom are hostile to surface-dwelling life. The Underdark is featured prominently in many D&D worlds and has also been called Khyber, which is noted as being home to a host of evil beings driven deep into the caverns at the end of the Age of Demons.
Inhabitants
The Underdark is extremely dangerous, especially to those not native to it. There are the usual dangers associated with caverns: claustrophobia, occasional poor air circulation, and getting lost. There is no light except for occasional patches of fluorescent fungus; most Underdark inhabitants either have highly developed senses other than sight or have developed infravision. Food can be extremely difficult to find, and much of the natural vegetation is poisonous.
In addition to those dangers, the Underdark also is the home of many predators and often-hostile races.
