Mars City Underground: Union Aerospace Subsystems

"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

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.

Summary
The player arrives at the Mars City Underground, where he is 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 is obvious that the battle is going poorly.

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

PDAs

 * Daniel Young. Obtained by killing Daniel Young.
 * Mark Ryan. Obtained by killing Mark Ryan.
 * Grant Baston. Found in Energy Stabilization Unit, near a storage crate.
 * Frank Delahue. Found in Energy Processing, on a table.

Characters

 * T. Brooks
 * Daniel Young
 * C. Rodgers
 * R. Fry
 * Ian Sinclair
 * Mark Ryan
 * Jonathan Ishii
 * Joe
 * UAC announcer
 * Sergeant Kelly
 * Bravo Team

Enemies
Note: Includes only enemies that can be fought and killed.

Weapons/Items
There are also 4 health stations on the level: two of them have 100 health in reserve, one has 50, and another has 80.

Cabinets
For the sources of the codes, see Doom 3 locker codes.
 * Cabinet #023: 531 Contains: Med kit (small), shells (small), 3 armor shards.

Secrets

 * In underground security (the first room), there are two armor shards on a hidden ledge in the corner to the left of the entrance where the pipes are (one needs to climb those pipes to reach them).
 * In the crane area, one can climb into crane cabin and find shells and two armor shards there. To reach the platform from which the crane can be accessed, the player needs to go downstairs in underground junction and find a cable that leads to that platform, which is located behind one of the pillars (there's a hidden med kit there). Once the platform is reached, the crane has to be turned to the load position by clicking on a nearby control panel. Note that after the invasion the crane becomes inaccessible because the control panel is broken. In the same platform there is a door to the maintenance storage. It's locked before the invasion, but can be opened after, from the security checkpoint (see below). There's another armor shard there, on the top of two boxes.
 * In the room with C. Rodgers, there are two armor shards hidden behind the stairs, and more on top of the machine that C. Rodgers repairs (can be access by jumping on crates then on pipes). These items are inaccessible after the invasion, due to the whole room being wrecked.
 * In the room with plasma collector, there are some armor shards below the floor plates.
 * In the area with folding bridges, the player can climb below said bridge using a ladder nearby, and access bullets, a med kit and armor shards.
 * In maintenance area PD1, there are shells and armor shards below the stairs that can be reached by carefully jumping onto the adjacent pipe.
 * In the surface area there are an armor shard, an adrenaline and a air canister lying behind the airlock the player first comes from. Also in surface area, one can climb onto the hill to the left of the opposite airlock to access a clip, armor shards and one more air canister.
 * Once inside the comm building, some armor shards and a med kit can be found on the shelves to the right of the airlock.
 * In the comm room where the action happens, some bullets (and a med kit on Recruit) can be found on the tables (hard to see because of darkness).
 * In the room with Frank Delahue's PDA, there are bullets, shards, and a med kit on a ledge near the table, and two more shards on a tall blocky machine nearby (both can be reached by stacking crates that are lying nearby). In the same room, the player can walk on a pipe to find another ledge with a med kit and shards.
 * In the room with a shotgun trap, there's a large pipe that can be climbed under, and behind it a med kit and two armor shards can be found.
 * In the room where lights go down, shells and a med kit can be found under the floor (a ladder can be used to climb back up).
 * In the room where the player first encounters an imp, there are a med kit, shells and armor shards on the table behind the stream of fire emitting from the gas pipe. To access it without taking damage from the fire, the player needs to input a failure code into the first control panel in the next room the player has go through, to shut down the gas leak. The code is 842, and it can be obtained by listening to one of the audio logs from Frank Delahue's PDA.
 * In the same room where the player can shut down the gas leak, the player can also use the third control panel to open a maintenance hatch nearby for some shells and another med kit
 * After the player passes through the catwalk above the folding bridge and enters the next room (with a health station immediately on the left), there are hidden bullets to the immediate right, on a blocky machine.
 * In the room with a storage crate on their way back to HQ the player is attacked by a shotgun Z-sec. After that, if the player goes a few rooms back, they will find a closet from which the Z-sec emerged and some shells and armor shards in it.
 * In the security office where the player needs to use a control panel to open access to the exit elevator, the same control panel has additional access options which can be revealed by pressing the "sublime access" button below. There the player can open the exit locker which contains the first machine guns, shotguns and clips, as well as unlock the door to maintenance storage. To access the maintenance storage the player has to go to the isolated platform with the crane (see above). The maintenance storage has a health station, bullets, early cells for the plasma gun, and armor shards (on the crates).

Trivia

 * On the Nightmare difficulty, you can kill the civilians to get an early charge for the Soul Cube.
 * At one point, the player will find a scientist being attacked by a zombie on a floor above them. If the player saves the scientist by shooting the zombie, later on they will find an opened (but broken) storage cabinet with health and armor in it. If the player doesn't save the scientist, the same cabinet will be closed with no way of opening it.