This level occupies the map slot MAP08. For other maps which occupy this slot, see Category:MAP08.
Walk behind the brick wall you saw in front of you while crossing the invisible bridge in the north of the starting area. Lower the middle, bricked part of the northern wall (across from the green armor and the tortured-corpse texture) to find an alcove (sector 101) containing a berserk pack.
Behind the yellow door, drop into the eastern sludge pit and run through the passage in the southeast to find a stone floor (sector 262) with a soul sphere on it. Use the teleporter to get back next to the yellow door.
Drop down into the blood pool in the east of the blue skull key. Push the back wall of the recess in the northern wall to open a passage (sector 250) leading to some stairs with nine health bonuses and a BFG9000 on it that you saw before from the room behind the yellow door.