UAC Mars Base

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

Background
Mars Base is the home of some of UAC's radioactive waste facilities. The Marine was stationed there for three years before being deployed to Phobos, where it's invaded by Demons. The Mars base is soon fell under attack where every human being were killed except the Marine, who single-handedly stopped the invasion.

Doom (SNES)
An image showing Mars reveals that a large section of the planet 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 The Lost Mission) are the same character. At the very least most sources indicate that the marine from Doom and Doom 3 are the same.

It is possible that after arriving on Earth, the marine was sent back to Mars. This would likely mean that Thy Flesh Consumed took place after his return to Earth on the Dropship and before Doom II started, rather than right after Inferno. If the marine from Doom 3 is the same as well, than it's possible that Doom 3 is actually telling the story of the marine's return to Mars post inferno.

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.