Mictlan was the extra-dimensional home of the Time Lord faction known as the Celestis. It served as a metaphysical bomb shelter removed from spacetime. It made its occupants immune to time winds or changes imposed on Gallifrey's past. (PROSE: The Taking of Planet 5) The appearance of this realm varied with the perceptions of its inhabitants. (PROSE: Alien Bodies) It was created by the use of block-transfer engines and computational matrices which also linked all the Lord Celestials who made up the Last Parliament.
The Bone Museum of Mictlan served as a library for agents of the Celestis. It held information on the Doctor. In addition, there was the Plain of Bones located in this realm. (PROSE: The Taking of Planet 5) The Celestis took those close to death to Mictlan. (PROSE: Autumn Mist)