So I haven't forgotten about this, but I kinda cornered myself. Even though I finished details for the rest of this deep, I never put an image of this room into a kind of electronic format. Figured I'd just make a diagram with letters the way we'd done for most of the corridors and such, but this one's a bit more complicated. I'll come up with something, but in the meantime, here's the verbal description...
Passing through the huge doorway, you find yourselves in a vast, stony chamber. Directly in front of you, the floor drops off. A few feet down, a rushing stream of some boiling, noxious fluid pours through the chamber. The only way across would be to leap across a series of thick posts that rise above the height of the boiling river. On the other side of the chamber a metal apparatus is attached to the floor. It looks like a valve, perhaps one that shuts off the stream...