This level occupies the map slot MAP03. For other maps which occupy this slot, see Category:MAP03.
MAP03: Poison Processing is the third map of Requiem. It was designed by Iikka Keränen, and uses the music track "Breath of Sin" by David "Tolwyn" Shaw. It is notable for containing simulated swinging doors at one point in the level.
At the southernmost sewage tunnel, in the section with the gargoyle-faced switch, enter the southwest tunnel, and travel on until you reach a dead end. There, you will find a few demons, security armor, and a berserk pack. Behind the berserk pack is a wall with three metal panels. Press on it to open the path back onto the stairs to the switch, and walk through it to trigger this secret. (sector 148)