Underdark

The Underdark was the vast network of underground caverns and tunnels underneath the surface of Abeir-Toril. It was home to a host of evil beings driven deep into the caverns at the end of the age of demons.
The Underdark was not one giant cavern under Faerûn, but rather, many huge networks of caverns and caves. As a result, it was not always possible to travel from one end of the Underdark to the other. The Underdark was divided into several domains that were similar to continents of the world above. While it was possible to travel from one place to another within a domain, separate domains tended to have very few passages linking them. The major domains of the Underdark were the Buried Realms, the Darklands, the Deep Wastes, the Earthroot, the Glimmersea, Great Bhaerynden, the Northdark, and Old Shanatar.