Underdark

The Underdark is the enormous system of subterranean caves, caverns, and tunnels located beneath the surface of Toriel.
Like many natural formations on the Material Plane, the Underdark have counterparts in the Feywild and the Shadowfell. In the Feywild, there exist the Feydark; In the Shadowfell, there exist the Shadowdark.

Type

Cave System

Description

The Underdark is not one giant cavern under Toriel, but rather many huge networks of caverns and caves. As a result, it is not always possible to travel from one end of the Underdark to the other.
The Underdark is divided into several domains that are similar to continents of the world above. While it is possible to travel from one place to another within a domain, separate domains tend to have very few passages linking them.
The Underdark's underground landscape varies from small subterranean fissures almost impossible for adventurers to crawl through to more conventional caves and caverns. But there are also wide expanses of deep water in which coral caves grow, blocks of ice that creatures hollow out and dwell inside, and cave-like spaces within masses of fungus or bone or else supported by bubbles of magical force.
This varied landscape is as dynamic as the surface of Toriel. Some areas are always changing over time because of underground magma rivers or the emptying of underground lakes due to cracks in the rock formed by earthquakes. This means maps of the Underdark are highly unreliable.