Civilians have reported that the hordes changed their Drinking Water Supply to a slime producing machine. The slime is now used to eliminate all human lifeform [sic] in this region. The civilians locked some of the doors to stop the demonic invasion, unfortunately it was too late.
Your mission target: find the keys, kill all demons and stop the slime machine. Then find the way back to your mission control and report what you've found. Don't come back without deactivating the machine!
Enter the northern and smaller of the two visibly yellow doors to reach the platform east of the power switch for the slime machine. There is a section of wall with an inverted cross and a pentagram drawn with blood. Open this wall to find two troopers on easy, sergeants on medium or chaingunners on hard skill levels, two stimpacks and a rocket launcher (sector 345).
After opening the path to the exit by using the switch mentioned in secret #1 and exiting the switch room, walk westwards. There is a lit section of the northern wall with a vertical strip of metal on it just before the stairs. Open it to find a compartment (sector 297) containing a barrel, four clips, four shotguns and four stimpacks. Coop players note: Opening this secret requires the possession of the yellow key.
After gotten ambushed by approaching the yellow key, open the western part of this cave's northern wall to reveal a passage up to the green armor.
Linedefs 197 and 229 do not possess middle textures, resulting in Hall of mirrors effects plainly visible when the player looks out of the window from the first hallway in the level.
Entering sector 338 (the one which contains a super shotgun, a stimpack and a box of bullets) without triggering linedef 1588 (Type 130 - W1 Floor Raise to Next Higher Floor (fast), Tag 23) first (raising that sector) causes the player to fall into an inescapable pit.