Mars City Underground - "The Mars City Sublevel was one of the first multi-purpose installations built on Mars. Housing storage, environment systems, localized energy production, communications arrays, and rudimentary research systems, the aging facility is vital to keeping Mars City operational."
Mars City Underground: Union Aerospace Subsystems is the second level of Doom 3, composed of the main area and an abandoned communications facility separated by a short walk outside. It is a small tutorial level covering a few more features in the game.
In the Communications Facility, things start to go awry. The action begins here, and zombies are first seen. The Imp is introduced at a cutscene.
For cells of the form x/y/z, x corresponds to Recruit difficulty, y to Marine, and z to Veteran/Nightmare.
Items that can only be acquired by using console commands are excluded.
Valve 1 Failure Code in Energy Processing: 842 (This will cause the fire to be extinguished and you will gain access to armor shards and ammo.)
Storage Cabinet #023: Supplies - 531
In the beginning of the level before the demon invasion begins, if you use the noclip cheat around certain areas, you will get to see Zombies, Z-Secs, and Imps.
You may rescue the scientist in the Processing Unit area; he says "Oh, thank you. I'm getting out of here" and disappears through a door. Later, you will see an opened storage cabinet. If the scientist is killed, this cabinet will be closed.