After the first door with blue lights bordering it, you may notice a very thin ledge leading to a platform, blocked by an exploding barrel. Destroy the barrel, and cross the ledge to find a switch. Press it, and bars will lower nearby. Drop into the pit, walk over the bars, and enter the curving passage to find a super shotgun and two boxes of shells in front of a nukage fall. Walk through it to obtain a computer map. (sector 250) To the west of the same location, you can jump onto a pillar with a soul sphere floating over it.
On your way to the switch that raises the bridge, there is a wide supercomputer with a switch on its west side. Flip it, and a nearby wall will open to unveil two medikits, four boxes of rockets, and a rocket launcher. (sector 241)
In the room west of the bridge, there is another narrow ledge you can cross to the southwest. Walk across it, dispose of the Hell Knight, and look to the southeast of the alcove to find a differently-textured wall. Open it to find four cells and a switch. (sector 247) Press it, and backtrack to the toxic pool east of where you found the bridge-raising switch to find a BFG 9000.
On your way to the blue keycard, you can find a section of metal paneling with piping on it, looking very conspicuous in comparison to the walls around it. Press on it to find a passageway containing a medikit and a plasma rifle. (sector 229)
As you enter the room with gray pillars in the northeast of the map, a wide alcove will open to release enemies. In the northwest of this alcove is a section of wall with cross-shaped paneling. Push it to find a bulk cell and security armor. (sector 352)