Coilfang Reservoir is a new dungeon in The Burning Crusade expansion somewhat reminiscent of Blackfathom Deeps. Also, Coilfang Reservoir is somewhat similar to Hellfire Citadel in that it contains 3 5-man "wing" dungeons, Slave Pens, Underbog, and Steamvault, as well as one 25-man raid dungeon, Serpentshrine Cavern. The new home of a growing army of Naga and various other water affiliated beasties Coilfang Reservoir is located within the Zangamarsh in Outlands

From the World of Warcraft Site:

The delicate ecology of Zangarmarsh has been thrown out of balance. Unnatural phenomena are corrupting and destroying the marsh's native flora and fauna. This disturbance has been traced to the foreboding Coilfang Reservoir. It is rumored that the leader of this mysterious edifice is none other than the infamous Lady Vashj.

Only you can discover her nefarious plans and stop them before it's too late. Coilfang Reservoir is divided into four areas, three of which are five-man dungeons (the Slave Pens, the Underbog, and the Steamvault), in addition to the 25-man raid dungeon, Serpentshrine Cavern.

The Slave Pens: Levels 62-64
The Underbog: Levels 63-64
The Steamvault: Levels 69-70