Mars City (Location)

Mars City is a base of the UAC located on Mars, constructed 25 years before the events of Doom 3. Highly trained Marines and a new unit called Mars-Security, regular guards of the base, were protecting it. Mars City houses the Marine Headquarters. All in the command of Sergeant Kelly. Mars City has plenty of office space, labs, In-Base Communications, a Break Room, a Medical Area, and it's own Armory. Mars City is the main facility on Mars, because it has the only boarding area, so you can leave or enter Mars. Mars City even has it's own Underground Maintance Area. Mars City even has it's own operational Sentry Bots. Mars City also has an elevator that leads to the Monorail Station. For fun, Mars City has a Arcade machine: Super Turbo Turkey Puncher 3.

Mars City Underground
Mars City Underground is what keeps Mars City operational. It has many Storge space, power, and it's own Communications Arrays. Most of the workers are freaked out going down there. Before you enter the area, there is a security checkpoint in control of T. Brooks. Mars City Underground also has a airlock that leads to the Mars Surface.

Trivia

 * When exiting the second level in Doom 3, when it says "You are now leaving" inside the elevator, it says "Click to exit Communications Transfer."