The Temple of Atal'Hakkar (aka Sunken Temple) was originally an end-game level instance. When WoW patch 4.0.3a was implemented the size of the dungeon was severely reduced, and the entrance was moved to a different location in the temple.
The area behind the old instance portal and the hallway leading to it still exist in the game! In this video we show how to access this location. Be sure to check it out soon. Rumor has it that flying under the world will be fixed with the next patch, which will make this a hard area to reach. The music is the Idjut Boys 'Riding The Storm' on Jazzy Sport Records.