This level occupies the map slot MAP28. For other maps which occupy this slot, see Category:MAP28.
MAP28: The Sewers is the twenty-eighth map of The Plutonia Experiment. It uses the music track "The Demons From Adrian's Pen" from Doom. The structure of the level is indeed reminscent to underground sewer lines with damaging "sewage" seeping every which way. It is notable for being a rather large stage with a lot of strong enemies to fight.
Other points of interest
After getting the yellow key from the library area, return to the map start area. Follow the hallway across the staircase mentioned in secret #1 to the east. In the room behind the yellow door, step onto the lit square platform to get the BFG-9000. Stepping onto the platform in coop game mode let players 1-4 get a BFG-9000 each. (This is done by Voodoo dolls that stand on lifts that are lowered when stepping on the aforementioned platform.)
From the start of the level, go through the opening to the left and continue westward, down a spiral staircase encircling a brown slime-filled room. Near the bottom of the stairway is a wall with scratches on it. It's a fake wall, so walk straight through and follow the pathway up to some goodies.
At the center of the level is a huge "+"-shaped tunnel with brown slime on the floor. At the end of the northern arm of this tunnel is a small bank of computers on the wall. Shoot it to open an alcove with a soul sphere in it on the opposite wall.
Just southeast of the "+"-shaped slime tunnel is a room with a square, raised nukage pool with square brown platforms in it. Run from one of these platforms onto the brown "fence" containing the nukage. Follow it along to the west and drop into the small pool of nukage against the wall. The green wall bordering this small pool is fake, and you can walk through it for another secret.
Make your way to the library area just southwest of the "+"-shaped slime tunnel. There is a broad bookcase at the south end of the largest of the library rooms; open it to reveal some goodies.
After riding the lift down to the library area (see official secret #4), look at the south wall. Push the black flag to open a secret room containing six super shotguns and six plasma guns.
There are actually two computer area maps in this map, which makes it impossible to gain 100% of the items, since the player can only pick one map per level. However, some source ports allow 2 to be picked up as a way to achieve the status.