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

 * Cabinet #023: 531 - Grant Bastion's PDA. Contains: Med kit (small), shells (small), 3 armor shards.

Secrets

 * There are many items hidden around on various unobvious places throughout the level. It's a good idea to check various ledges, below stairs, etc. There are also items scattered around in surface area, including on the top of a small hill.
 * In the room where the player first encounters an Imp, there are some supplies 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 additional supplies.
 * 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 supplies 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 in the game, as well as unlock the door to maintenance storage. To access the maintenance storage the player has to go to the place with the crane, walk downstairs and then, behind the pillar to the left, find a cable that leads to an isolated platform far away, and walk along the cable. There's a med kit near where the cable is connected to the pillar, to make it easier to find.

Trivia

 * On the Nightmare difficulty, you can kill the civilians to get an early charge for the Soul Cube.