The Underdark is a well known but poorly understood portion of the world consisting of caves, catacombs, dungeons and the cities of unknown number. All manner of foul creatures are known to haunt the Underdark including Derro, Drow, Duergar, Ithilids and more. Surface dwellers who enter the Underdark typically find themselves lost in a matter of mere hours. Even the deepest of Dwarven Holds only scratch the surface of the Underdark. Rumors abound about the deepest levels of the Underdark but its unclear if even the denizens of the Underdark fully comprehend the depths.