Mictlan

Mictlan was the extra-dimensional home of the Time Lord faction known as the Celestis in order to serve as a metaphysical bomb shelter that was removed from spacetime thus allowing its occupants to be immune from time winds or changes brought onto Gallifrey's past. (EDA: The Taking of Planet 5) The appearance of this realm varied upon the perceptions of its inhabitants. (EDA: Alien Bodies) It was created through the use of block-transfer engines and computational matrices which also linked all the Lord Celestials that made up the Last Parliament together.

The Bone Musuem of Mictlan served as a library for agents of the Celestis and included information on the Doctor. In addition, there was the Plain of Bones located in this realm.

It was destroyed due to the actions of the Hermit, a renegade Celesti and the actions of the Fendahl Predator. (EDA: The Taking of Planet 5)

Behind the scenes
Mictlan was the name of the Aztec underworld.