While inside the exit room, shoot at the middle of the west wall to open access to a hidden area. (sector 251) Inside, you will find two sets of four shotgun shells, a berserk pack in front of a switch, and a switch nearby a pillar blocking access to an energy cell pack. Flipping the switch behind the berserk pack enables access to the cell pack, as well as a hallway accessed from the center of the area ending in a switch. Pressing this switch lowers a pedestal with a megasphere on it in the southeast of the area. Pushing the switch near the pillar blocking access to the cell pack opens access to a BFG9000, as well as a doorway in the northwest leading to another switch near the center of the area. Activating this last switch opens the nearby wall, revealing five shotgun guys and four rocket launchers, and also unlocks access to the megasphere.
In the PC version, the "catwalk" bridge to the exit lowers as the player dashes across it, lowering into a nukage pool with no way to escape, but in the PlayStation version of Final Doom, the bridge does not lower into the nukage, and two teleporters are on either side of it.