1. At the very south of the level is a white room with a plasma rifle placed behind a small barrier. To the east are monitors with red displays. Push them and the barrier will drop, allowing you to get the plasma rifle.
2. Just before you descend the stairs to the sewer, there is a a mass of corpses in front of a computer. Activate this computer to find a secret passage lined with bonuses.
3. In that same passage, there is a flashing arrow on the floor and ceiling pointing towards the east wall. Activate the section of the wall it points to and you will find the red skull key.
4. Return to the nukage room, and make your way to the southeast of the room (I recommend you activate the bridge from the southeast to the southwest, and use the teleporter in the southern wall to get to the southwest, so you can use this bridge to get to the southeast quickly.) Open the east door, and with the red skull key in hand, open the red door in the southeast of the next room. It leads to a passage made of red marble, which leads to a room with a single switch. Activating this raises a secret bridge in the southwest of the nukage room, allowing you to access a door leading into the ventilation system.
5. Once you're in the ventilation shaft, head through it until you find a fork in the passage, with a tunnel to the left and right. Take the right page to find a room containing health and a mega armor.
6. Once you get the mega armor, head back to the fork, look directly to the south, and open the door in front of you to find a power core, around of which is the BFG and a switch. Activate the switch to open the way out of this room.
7. Activating the switch in the previous secret will also open a door in the crater outside, to the south of the room where you started the level. Open the door beyond it to find a soul sphere, some ammo, and a platform accessible by jumping across a pillar.
8. Beyond that platform is a doorway which is blocked by bars. Shoot both of the demonic faces around it and the bars will open. Enter this room to find a strange gateway with the Community Chest insignia on the top and bottom. This is the secret exit; stepping in it will take you to MAP31: Mortar Combat.