Khyber

Khyber is the area beneath Eberron and is said to be the body of one of the three Progenitor Dragons that created the world. It is a massive subterranean realm that is filled with fiends, Aberrations, Daelkyr, bound Overlords, and other monsters. While the upper layers of Khyber are stone and dirt, the lower levels are a vast array of demi-planes with architectures and denizens that defy description. Khyber is sometimes called the Underdark by people who think all drow look alike.