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. It is a small tutorial level covering a few more features in the game. AREAS IN MARS CITY UNDERGROUND: