The Underdark is a vast series of caverns deep below Stone, stretching under several continents and oceans. It is home to races such as the Drow, the Duergar, among many others. As it is a place away from the sun and the eyes of most gods, evil prospers there, and monsters run rampant. Many a dwarven mining expedition has been lost without a trace after breaking into the Underdark.
There is no main entrance into the Underdark from the surface, like Dwarven citadels tend to have, but rather small hidden caves offer entrance to and from the deep across the continent of Stone.
Very little is known of the Underdark, only that it is a place of great evil, but also great adventure.