There are several new instances in World of Warcraft: The Burning Crusade.
* Auchindoun was formerly a sacred burial site for the draenei. It then became the fortress of the Bleeding Hollow orc clan. It was nearly destroyed by Alliance forces during the invasion of Draenor. The Shadow Council almost destroyed the tomb and the surrounding areas when they summoned Murmur, who is said to be the primordial essence of sound.
* Caverns of Time lie below the Tanaris desert in Azeroth. It is where Nozdormu the Timeless One, head of the Bronze Dragonflight and Dragon Aspect of time, makes his home. The Caverns of Time is actually comprised of three instances, in which players will be able to experience significant events of Azeroth's history and prevent them from being altered by a mysterious foe: Thrall's escape from Durnholde Keep, Medivh's opening of the Dark Portal, and the Battle of Mt. Hyjal.
* Coilfang Reservoir is a subterranean pumping station operated by Illidan's naga forces. Lady Vashj herself personally oversees its operation.
* Gruul's Lair is the lair of the massive ogre demigod, Gruul.
* Hellfire Citadel was formerly an orc fortress, and housed the warriors of the Shattered Hand and Shadowmoon clans. More recently, Illidan has converted it to his own uses, and is breeding an army of fel orcs within its walls.
* Karazhan was once the shining ivory tower of Medivh, the Guardian of Tirisfal. After Medivh's death, however, the tower and the land around it were blighted and cursed. The powerful nexus of ley energies beneath the tower still remains, however, as well as the myriad of treasures accrued by the powerful wizard.
* Tempest Keep was created by the Naaru to serve as both a fortress and a means of transportation. However, after arriving in Outland and leaving the keep, Prince Kael'thas and his blood elves stormed in and took over. It now serves as the prince's base of operations in Outland.
(post courtesy of Wikipedia)