You need to get there from pipes 1 - using the room to the NE of the zone. You get to an area where you are given various quests which eventually allow you to progress further.
To get to the NE room you need to pull each of four levers scattered around the pipes level - once each. Thise are the levers that don't seem to do anything near where they are. They open the doors to that NE rooom.
