UAC Mars Base

Mars Base is a location mentioned in Doom and Doom 2 manuals. It is a UAC facility on Mars.

Background
Mars Base is the home of some of UAC's radioactive waste facilities.

This is were the Marine of Doom 1 was stationed after he was sent to Mars after attacking an officer on Earth. He spent some three years there before being deployed to Phobos to stop an invasion there.

In Doom 2 the backstory is that the Marine or a Marine stopped an invasion of Mars Base before flying back home in a drop pod on shore leave. He was the only survivor. Only to discover Earth was invaded. This parallels the plot of Doom 3 (and Doom RPG).

Doom Snes
An image showing Mars shows that a large section of Mars has apparently been colonized.

Doom 3
See also UAC Mars Research Base

Behind the scenes
The manual for Doom II tells of a completely different set of events than occur in both the endings of the original Doom or Ultimate Doom which took place on Phobos and Deimos instead, and lead to the Marine in those games teleporting to Earth through gateways.

Some Fanon believe this to mean that the marines in Doom 1 and Doom 2 are completely different individuals who reached Earth by different means (Mars hero by drop ship, and the Phobos/Deimos marine by teleportation). With one hero having battled on mars and the other battled through Phobos and Deimos instead.

To add to the confusion some of the later manuals and websites also suggest that the marines in Doom 1, 2, 3 (and sometimes those of Resurrection of Evil and Lost Episode) are the same character.

Who knows we but the differences in these characters might have inspired characters such as Doom (character) and Phobos (character), B.J. Blazkowicz III and Stan Blazkowicz as separate characters in Quake and Doom RPG respectively.