The Waterworks
You must be at least level 60 to enter the waterworks. I would recommend 3-4 people for this dungeon as it is a very hard dungeon. There are 5 bosses. This dungeon is a cheating dungeon if you do not follow the rules you will almost certainly die. You can get your waterworks armor which is some of the best armor in the game. When you complete the waterworks you will the Waterworks Warrior badge. Here are the cheats that the bosses use.