To the right of the switch is a slightly miscolored wall, interact with it and enter this secret room. Flip this and grab the Backpack Full of Ammo. Around the corner, you will come to a platform with a switch on it. If you face the entrance and look to your left there is a doorway in the acid. In here is an imp, so kill him, then pick up the Rocket Launcher. You will see a similar wall as before, interact with it and go through here. Go back up the stairs (quickly) and take the left path. Save as much as you want as you go through the level, so if you take too much damage at any part you can just reload. Make a save, you will want to end the level like this for an achievement. At this point, you should have 200% health and 200% armor (if you are close just pick the small armor and health pickups as we progress through the level. On the right side of this pillar is a switch, interact with it and take it. Continue through here and you will come to a pillar with a Supercharge on it. When you get through, kill the imp at the bottom of the stairs. If you did not get there in time, just go back down the stairs and try again. Go up to this before the platform raises and interact with it. You will see an exposed wall (pictured below). Take this path up some stairs and follow the path to the left. There will now be a new path across from the door you entered this room from. At the bottom of the stairs go left to find a button.

There will be a couple of enemies in here, so kill them. Then next follow the hall to the right and take the first left you can down some stairs.

Doom nuclear plant secrets