U is for Underdark

The underdark in the simplest terms as underground campaign settling. To expand on this definition lets parse the word "Underdark". Under meaning below but it also has lesser connotations which evokes the underground. Dark meaning absence of light which has it's own associations such as hidden and blackness. The underdark is seemingly lightless realm of empty deep caves and tunnels. For our gaming uses, it is a dark analog of the surface world. Filled with illuminating fungus and lichen. Inhabited by prowling ravenous beasts. Dominated by continuously warring humanoids trying to survive in a land of scarcity. Races below parallel those from above but are honed by the need to survive in a harsh environment. The need to survive and compete has made them isolationist, aggressive, cunning, territorial, and greedy. They also have adapted their bodies and developed new skills to function in near darkness and in a barren wilderness. Despite these challenges they have over ages created farms and cities. Precarious at best a small change in the environment or a stronger group may destroy the culture. Ruins of lost civilization dot the under dark. Discovering let alone exploring the underdark are immense challenges. Escaping it may be more of difficult then getting in. The underdark is campaign setting unto itself and should be treated as such.