Doom Wiki
This article is a stub. Please help the Doom Wiki by expanding it.

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.

In the Communications Facility, things start to go awry, and the invasion begins. The action begins here, and Zombies are first seen. The Imp is introduced at a cutscene.


  • Underground Access Elevator
  • Underground Security Checkpoint
  • Underground Junction
  • Maintenance Area CS1
  • Convergence Chamber 1
  • Energy Stabilization Unit
  • Convergence Chamber 2
  • Energy Processing
  • Energy Delivery
  • Maintenance Area PD1
  • Underground Garage
  • Airlock MC-2a
  • Mars Surface
  • Airlock CT-1c
  • Old Comm Storage
  • Old Comm Engineering Elevator
  • Old Comm Engineering
  • Old Comm Main
  • EP Aerobic Stabilizer
  • EP Control Unit
  • MCU Storage




Monster Count
Imp 7
Zombie 14/14/15
Pistol Z-Sec 3
Shotgun Z-Sec 3/3/4
Weapon Count
Flashlight 1
Pistol 1
Shotgun 4
Machine Gun 2
Ammunition Count
Pistol ammo (small) 15/15/13
Shotgun shells (small) 13
Shotgun shells (large) 2/2/1
Machine Gun ammo (small) 2
Machine Gun ammo (large) 1
Plasma cells (small) 2
Misc. Items Count
Adrenaline 1
Small medkit 23/23/15
Large medkit 2
Armor Shard 83/83/73
Security armor 3
  • 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.

Cabinet Codes[]

  • Storage Cabinet #023: Supplies - 531
  • 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.)



  • 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. These are in the areas past Energy Processing.
  • You may rescue the scientist trapped in the Energy Processing 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.
  • When the invasion starts, a Pistol Z-Sec and a Maintenance Zombie appear in the Communications Facility. This is rather strange, as Dr. Ishii appeared to be the only person in the building.
  • When activating the bridge in the Energy Delivery area, a scientist can be see running across the walkway out of the Energy Processing area shouting "No! I don't wanna die!", before disappearing through the other door. He has a larger amount of health than other human NPCs, as he is able to survive several pistol shots. A BFG-9000 shot (obtained using cheats) will kill him quickly.
    • This scientist is a Joe, as seen when noclipping to get closer to him.