Coilfang Reservoir

Coilfang Reservoir is a large artificial cavern located in Zangarmarsh. Under Serpent Lake is a subterranean pumping station, operated by the sinister Naga. The dungeons are found in this underground area, and consist of gigantic caverns with huge pipes descending from the ceiling powered by titanic crystal capacitors. While the entrance to the reservoir is underwater, the instances themselves are only partially submerged, similar to the Blackfathom Deeps.

Four instances comprise Coilfang Reservoir: three 5-man dungeons and one 25-man raid. Similar to Hellfire Citadel where the dungeons are designed for differing levels, The Slave Pens is designed for levels 61 to 64 and The Underbog is designed for levels 62 to 65, while both the 5-man The Steamvault and the 25-man Serpentshrine Cavern raid are designed for level 70 players.

Official info
Worldofwarcraft.com says the following about the Coilfang Reservoir:

''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.''

Coilfang Reservoir is the name for a naga-dominated compound that lurks under Zangarmarsh's deepest waters. The reservoir, punctured throughout by massive open pipes and pistons, seems big enough to house armies - and, yet, its purpose isn't strictly military.

Here, concealed by the cold fathoms, Lady Vashj's naga ply their resources - the exotic flora and fauna of the cavernous Underbog, the Broken who toil in their claustrophobic Slave Pens, and the arcane machinery they've constructed in the Steamvault - towards a catastrophic goal set forth by Illidan the Betrayer. Unopposed, they will drain Zangarmarsh's waters, and claim dominion over the dry bog that remains.

The Slave Pens

 * Level 62-64 mobs
 * 5 player groups
 * Bosses: Mennu the Betrayer - Rokmar the Crackler - Quagmirran

The Underbog

 * Level 63-65 mobs
 * 5 player groups
 * Bosses: Hungarfen - Ghaz'an - Swamplord Musel'ek - The Black Stalker

The Steamvault

 * Level 70-72 mobs
 * 5 player groups
 * Bosses: Hydromancer Thespia - Mekgineer Steamrigger - Warlord Kalithresh

Serpentshrine Cavern

 * 25 player raid
 * Bosses: Hydross the Unstable - The Lurker Below - Leotheras the Blind - Fathom-Lord Karathress - Morogrim Tidewalker - Lady Vashj