When the level begins, shoot the computer panels behind you with any impact weapon to find an alcove with a chaingun and armor. (sector 171)
In the southeast room that opens into the lava cavern, push the computer mainframe at the north, and it will rise to reveal a radiation shielding suit. (sector 219)
At the south of the lava cavern, you can enter a cave. When you walk into the next room, a wall will open. In the passage that is revealed is a candle sitting in front of another wall. Press on the wall behind it, and it will open, allowing access to a berserk. (sector 192)
After entering secrets #3 or #4, return to the middle of the lava cavern to find that the eastern wall has opened to let out nine spectres. In the room where they reside is a switch. Activate this switch to lower a teleport pad, and step on to be warped into a cavern in the south of the level containing three bullet boxes, three energy cells, an energy cell pack, and two medikits. (sector 234) Step back onto the teleport pad you landed on to return to the room you warped in from.
The southern closet in the computer room contains a plasma gun and a medikit. To the west of them is a wall with a misaligned texture; open it to find two energy cells. (sector 103) On skill 3, there will be a energy cell pack in there as well, while on skill 2 or lower, there will be two.
The space in which the yellow key is found counts as a secret. (sector 193)
To the northwest of the yellow key is a chainsaw, which also counts as a secret. (sector 195)
In the storage room southeast of the cafeteria, there is a switch at the north of the room. Acitvate it and a stack of crates in the southeast will act like an elevator. Take the elevator to find two small alcoves, one of which contains a megaarmor. (sector 167) If you remain on the lift when it rises, press the switch to your north to lower it again. Optionally, you may also press the western switch in the same location to enable access to the supercharge in the first room.
To the southeast of the blue key mountain is a teleporter behind a wall. Entering it warps you into a teleporter bay east of the lava cavern, where you can grab armor, and open a door to find a box of rockets and an opening back into the cavern. Instead of going back into the cavern, however, re-enter the teleport pad you landed on, and you will be taken to another cave in the south of the level, with two boxes of shells and a medikit. (sector 236) On skill 3 or lower, there will be an additional medikit. Step back onto the teleport pad to return to the blue key room.
In the southwest of the penultimate room is a skull switch. Push it and head northwest to see that the wide crate has lower to reveal a corridor. Walk past the "no permittance" signs at the entrance, and a doorway will open in the northeast. Run in to find three health bonuses and a switch. Hit the switch, take the lift up, and head through the passage to find a box of rockets and an energy cell. Walk through the northeast wall of the room to find a switch. Press the switch, and the wall in front of you will open to reveal two alcoves. In the northern alcove is a backpack. (sector 165)