Mars City Underground: Union Aerospace Subsystems


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Doom 3 maps
Map name: game/mc_underground
"All units. This is Sergeant Kelly. We're under attack by an unknown enemy force. Fall back to Marine HQ to regroup! I say again fall back to Marine HQ and await further orders!"
― Sergeant Kelly [source]

Mars City Underground: Union Aerospace Subsystems is the second level of Doom 3, composed of the main area and an abandoned communications building separated by a short walk outside. It is a small tutorial level covering a few more features in the game.

In the Communications Building, things start to go awry. The action begins here, and zombies are first seen. The Imp is introduced in a cutscene.


Spoiler Warning: Plot details follow.

The Player arrives at the Mars City Underground, where he's greeted by T. Brooks, who gives him access to some basic gear: armor, a flashlight, and a pistol. The Player then heads through the facility to the old comm station where the missing scientist is believed to have gone. Along the way, he runs into several workers, all of whom seem to be nervous and on the edge. He also picks up Grant Baston's PDA, which contains a code for a locker containing supplies. After a brief walk across the Martian surface, he reaches the old comm station. Inside, he finds the missing scientist - Jonathan Ishii - trying to send a message to the UAC.

Ishii frantically tries to explain that he must send a warning out while there is still time. Before he can explain further, another teleportation experiment is conducted at the Delta Complex. Something goes horribly wrong, however. The portal stabilizers suddenly fail, causing a series of explosions throughout the base. At the same time, mysterious ghost-like skulls come flying out of the portal, possessing many workers and security guards. Ishii is possessed and transformed into a zombie, which attacks the player. After killing the zombified Ishii, along with a Z-Sec that bursts in moments later, the Player receives orders over the radio from Sergeant Kelly to fallback to Marine Command to regroup.

The Player must backtrack his way to the elevator at the beginning of the level. At one point, he is forced to take a detour through Engineering, and must use Frank Delahue's PDA to access the area. Along the way, he is attacked by zombies and Z-Secs, and eventually Imps. Fortunately, the Player also picks up a couple of better weapons - the shotgun and the machine gun. Throughout all this, the screams and panicked shouting of fellow marines can be heard over the radio. Sergeant Kelly is heard trying to coordinate a defense, but it's obvious the battle is going badly.

Eventually, the Player returns to the elevator and rides it back to Mars City.

Spoilers end here.


The trip to Jonathan Ishii is a simple follow the corridor sequence.

When you meet him, hell breaks loose, with Johnathan being transfored to a zombie within a few seconds. Kill him quickly, and turn your attention to the door on the left, watching for the Z-Sec that exits and attacks. Head to the elevator, and go to the first floor. Exit the airlock, and renter the airlock in the other building.

When you return to the first building, there is a zombie to the right on the first floor. Ascend the stairs, and open the door to engage the zombie that attacks at point-blank. Head up the stairs and into the room with the bridge. Kill the zombie there, and extend the service bridge to cross it. There is a Z-Sec that attacks from an alcove just past the bridge. Enter the door.

A Z-sec will take cover behind a crate, and a zombie will approach froma crawlway in the left. Search this room for the Frank Delahue PDA to unlock the door and enter.

A scientist calls for help from the platform above, and you can kill the zombie as it approaches. There is also a second zombie around the corner to the right. Go through the door on the left.

Watch the floor, it's exposed machinery. If you want armor and a shotgun, there is some to the right, but it activates an ambush where the floor lowers and zombies attack from the sides. Climb the ladder to return. Exit through the door on the left corner of the room.

Head down the corridor, and engage the Z-Sec and two zombies at the top of the stairs. Walk backwards past the stairs to engage the flanking zombie that attacks from a closet, and turn forwards to get the non-flanking zombie. There's a health station at the end of this corridor, which you can use to heal.

Past the door is a cutscene with a new enemy. Two shotgun blasts should be enough to destroy the Imp. Continue through the door.

The next room (where you first found the scientist calling for help) has an imp coming around the corner, followed by two zombies. At the end of the corridor, there is a zombie hiding behind a pillar with planted armor shards, and an imp that attempts to flank you from behind.

Nothing in the platform overlooking the service bridge room.

A health station to the left with 50 health. But on the ground floor, there is an imp that jumps from behind the first corner, followed by a zombie behind a pillar. Continue to the next room.

Nothing in the plasma filter room. Continue left (as the door on the right was busted by an imp.)

Nothing in the convergence chamber 2, but an Imp will perform a jump attack shortly after opening the exit door. After defeating the imp, walk backwards into the next room.

A z-sec attacks from the convergence chamber. Once eliminated, you can look at the storage cabinet, code 531. When you approach, there is one imp from the crawlway on the left path. Proceed to Convergence Chamber 1.

Convergence Chamber 1 is only a jump scare.

Where you met the mysterious scientist that wouldn't tell you what's going on, an imp will break out of the stairs on the left.

You should now return to the crane room - an imp climbs from the rail on the opposite side. When you approach the stairs, the crane will break, dropping heavy materiel onto the stairs. Head up, and an imp will attack from the security checkpoint, which you should enter.

Defeat the Z-Sec stationed there, heal from the health station, and head to the elevator. The last enemies are one Z-Sec coming from the opposite side, one Imp that drops from the ceiling in the elevator room, and one pistol Z-sec in the elevator itself.




Former Humans




  • Cabinet #023: 531 - Grant Bastion's PDA.


  • When you enter the "Energy Processing" room where you saw the scientist complaining to the maintenance guy earlier, after you kill the imp, there's a zombie hiding behind one of two pillars. Behind those pillars is a closet with shells and armor shards that never opens. The only way to see it is with the "noclip" cheat.
  • On nightmare difficulty, you can kill the civilians to get an early charge for the soul cube.

External links

  • Walkthrough video by WoopzillaWalkthrough (part 1; part 2) on YouTube. Veteran difficulty; part of a full game run.