THERE IS A REASON MANKIND FEARS THE DARK.
The Underdark is featured prominently in the campaign settings World of Greyhawk and the Forgotten Realms. The concept of a dungeon that spanned a planet was first introduced by Gary Gygax in his D-series of game modules and at the end of the G-series. The Underdark is described in detail in the 1986 manual Dungeoneer’s Survival Guide, by Doug Niles. It is also a part of the Eberron campaign setting, in which it is called Khyber and is home to a host of evil beings driven deep into the caverns below.
The Underdark is described as a vast subterranean realm inhabited by drow, mindflayers, aboleth, and other strange, sinister creatures. It is a place where few humans go and from where even fewer return. It extends well past the dungeons made by surface dwellers, encompassing all the myriad caverns, tunnels and complexes that exist for all intents as a parallel world beneath the surface.
One of the best TSR boxed set campaigns is NIGHT BELOW an adventure for four to eight first-level characters. The boxed set is immense and very detailed. The adventures literally allow a Dungeon Master to take a gaming group throughout different layers of the Underdark, introducing the unique social structure there. Assuming that the characters survive the myriad dangers, they will be 14th level by the time they finish this campaign. However, finishing the campaign would take years, unless you’re planning on running some of those famous Monte Cook marathon sessions.
The Underdark’s environment varied vastly from the surface world, making adventuring through it a dangerous expedition. Just like trekking through a desert, adventurers traversing the Underdark’s passages needed to be well prepared.
The Underdark was mainly without any source of light save for the occasional luminous rocks or fungi. Large portions of it were also without food or water, making well-stocked food supplies a must-have when exploring uncharted terrain underground.
Lack of light combined with a tomb-like silence made traveling something only for the strong-hearted. The nerve-wracking silence made even experienced adventurers grew agitated after a few days.
Just like deep dungeons, the air could grow stale and even poisonous in the Underdark. It could contain explosive gases from underground volcanic activity or other deadly gases bubbling out of deep fissures in the rock. Bad air and poisonous fumes could collect in portions of tunnels and caves more or less isolated from the main vaults and caverns. Siphon-like tunnels holding water can also trap these gases for a long time.
The Underdark was extremely dangerous, especially to those not native to it. There were the usual dangers associated with caverns: claustrophobia, occasional poor air circulation, and getting lost. There was no light except for occasional patches of fluorescent fungi; most Underdark inhabitants either have highly developed senses other than sight or have developed darkvision. Food could be extremely difficult to find, and much of the natural vegetation is poisonous.