Go north, then east. Run into the northern room, and dash west. Jump into the nukage to the north, and grab the yellow key. Take the lift, and open the yellow door to the northeast. Head through the cave, climb the rocky stairs, and grab the red key. Backtrack through the yellow door, head outside to the southeast, and open the red door. Rush toward the switch at the north, and a barrier will lower in front of it; at the same time, a compartment filled with imps will open to the east. Flip the switch inside to open the barrier, and then trigger the northern switch.
Other points of interest
To the east of the yellow door is an elevator. You can flip the switch nearby the entrance to the nukage area, and then run into the lift before it rises. Inside are a few crates, a stimpack, a chaingunner, and a super shotgun. Be careful as you climb onto where the super shotgun is, as a revenant will teleport in the lift immediately afterward.
Not far from the beginning of the level is a pile of grey and tan crates with a blur artifact on top. There is a small grey crate between two larger ones. Stepping on the small one causes it and a larger crate nearby to sink into the floor. Run over to the larger crate, and stand on it before it rises again. Ride it up and hop across to the now-accessible blur sphere. (sector 68 in the first release; sector 67 in the second.) Be careful as you snag it, as an imp will teleport near you.
In the southeast corner of the nukage pit, a soul sphere rests on top of a column. There is a small red button behind the falling nukage on the west side of the room that lowers this column. (sector 49) Note that obtaining the soul sphere will cause alcoves to open nearby, revealing chaingunners.
Areas / screenshots
A shotgun can be obtained outside of the starting area.
The player stands at the opposite side of the yellow key, with former humans laying dead on the floor.