Eradicator/Ionic Decay (DBonus)

This level is a good example of level authors taking a more free-form and for-fun approach. Secret levels in general are a good opportunity for this sort of "freestyling" and for trying things that wouldn't be thematically appropriate in the normal mission. And like many secret levels, this one is replete with ammo, powerups and items.

Three-level area
You start next to the top of a smallish but tall room with three levels of walkways. Go to the bottom level, find and activate two switches. The one by the lift activates the lift and opens the door above; the other opens two small side areas on the second level. One of these areas has a switch that unlocks the door next to where you started the level. (There are good items inside there, and it counts as a secret area.)

Red waste
Take the door at the top of the lift to the next area. Near the door is what looks like a lift: the security block; you can see around it to a lower room. Inside, you'll have your first encounter with Droppers: those hovering creatures with the dangling tentacles. They're what's making that mechanical whirring noise. Be on guard immediately whenever you hear them and don't fight them from too close up: their flesh and gibs are extremely hazardous and will cause a ton of damage, and they have a nasty habit of sneaking up on you. (Tip: fire some of your #6 ammo down the crack. This will usually waste the Droppers while they're confined there. Just back up right away so their gibs don't hurt you.)

Go down the L-shaped flooded passage. You learn the red lava-like stuff is a byproduct of decayed Mazrium. At the corner is a switch that lowers the security block. There's an Invulnerable in that lower room; the teleporter takes you back to the three-level room.

Small courtyards
Go through the L-shaped passage, and down to another tall room, open to outside, with an upper level of walkways. The two doors are locked right now. Go to the upper level and hit the switch in the nook. This unlocks one door and opens a compartment on the upper level (watch out for the Dropper!). Go to the door that's unlocked, and hit the switch inside to unlock the other door and open a second compartment on the first level. This also activates a hidden lift on the floor between this second door and the entrance to this room, that goes down to a secret area. (There's a message saying another switch is revealed, but there are no additional switches in this area or earlier parts of the level. This message is probably an uncorrected leftover from an earlier version during the level's development.)

Take the second door, down a corridor, to a tall flooded area with platforms and walkways. The blocked teleporter in the room to your right is the level exit. To the left is a small but tall courtyard. First, at the far end of this flooded section there's a Geosurvey down in the red waste stuff.

Go to the courtyard, and up the ramp to the next room, and hit the switch in the back. This opens a side room in the courtyard, with a newly-opened passage that leads back to the corridor before the red-waste-flooded area. Hit the switch inside that room and a second side room in the courtyard opens. Hit the switch in that side room, and a big Classic Quake-style teleporter in the wall opens nearby, leading to that overhead platform in the courtyard, which is a secret area with a full-health.

Exit
Take the corridor back around to the flooded section, and jump to the exit room. Some side areas in here will have opened, with a small ambush and additional items. You've now seen everything in this level. When you've collected everything you can carry and have explored where you want, exit the level to resume your main mission.