The Underdark is a literal underworld to the surface world for the Dungeons & Dragons role-playing games. It is a vast subterranean network of interconnected caverns and tunnels stretching beneath entire continents.
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 darkvision (infravision in earlier versions of D&D). Food can be extremely difficult to find, and much of the natural vegetation is poisonous.