[9-25] E4M1: The Sewage System

At the top of the elevator, turn right and shoot the wall with the computer monitor to reveal yellow armor and quad damage.

In the room with the biosuit near the exit slipgate, shoot the (+) symbol above the door. This opens a wall outside the room containing a teleporter that leads to a yellow floorplate near the gold key door.

Just beyond the gold key door, on a ramp, a wall behind you will open, releasing dogs. To your left is a ledge with quad damage. Above you are three hanging fluorescent lights. Run up the ramp and jump into the third light. You must be running up the ramp towards it to hit it. This will open the way to the quad damage.

In the last underwater room before reaching the exit slipgate, look up and to your left to find a secret room.