The Underdark is a vast complex of tunnels, caverns and passages underneath the surface of the Known world. It used to be ruled by dwarves until the dark elves drove them out. It is divided into three parts - Upperdark, Middledark and Lowerdark.
Geography
→ World above - the name the Underdark inhabitants use for the surface
→ The Upperdark - closest to the surface, first three miles below
→ The Middledark - three to ten miles below the surface, contains most of the cities and the seas
→ The Lowerdark - located ten and more miles below, it is said to be the most dangerous part
Environment
With no sun the only light source is either luminous rocks or fungi. The uninhabitable parts lack a source of food and water. With darkness and silence the traveling throughout the Underdark can get to one’s nerves. Some caves get to have poisonous, or even explosive gas. Outsiders are not advised to stay in the Underdark for too long, as they will eventually hear the Calling. Once being exposed to this, the victims of the Calling are made to carry out tasks, while having no control over their body and mind. Those born and raised in the Underdark are the only ones being able to resist the Calling.
Climate
As it is with caves, the temperature remains chilly most of the time, all depending on the depth of the area and its location though. The rivers and seas are cold, especially for non-inhabitants.