[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 (a portal to another area), shoot the (+) symbol above the door. This opens a wall outside the room, revealing a teleporter (a device that instantly transports you) that leads to a yellow floorplate near the gold key door.

Just beyond the gold key door, there is a ramp. Behind you, a wall 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 the light, which will open the path to the quad damage.

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