Mars

Mars is the fourth planet from the sun in the Solar System. A barren, wind-swept, rocky desert world with a tenuous carbon dioxide atmosphere and very little water, it figures as a significant setting in many versions of the Doom canon, where humankind has begun the process of its colonization.

Classic Doom
Doom's action takes place on Phobos and Deimos, the two moons of Mars, although in the introductory story the protagonist is initially stationed on Mars itself, where the UAC has set up nuclear waste disposal facilities. The military, of which the player character is a part, has loaned the Martian moon bases from the UAC to perform research on teleportation technology.

It is often assumed the setting of the game is on Mars, but in fact none of the events take place on the planet. In fact, the player never ventures to the surface of the planet in any of the games or episodes. Doom 64 may take place on the planet's surface, but the game is ultimately vague about its setting and refers only to it being under a policy of "planetary quarantine," maintained due to apocalyptic levels of radiation.

Doom 3 and expansions
In Doom 3, Resurrection of Evil, and The Lost Mission, the action is set directly on Mars, in the fictional Mars City. The player is required to venture onto the barren surface of Mars a number of times during Doom 3, and must fight off demons while topping off his oxygen supply via collectible tanks. Although the planet is portrayed as being realistically inhospitable, a decision was made early on to increase Mars' gravity to match Earth's for gameplay reasons.

In the continuity of Doom 3, Mars once hosted a population of highly advanced humanoids who created a complex civilization and controlled a technology described simply as "mystic" by UAC scientists. Remains of this ancient civilization were discovered by the UAC during construction of the first underground Mars base at Site 3. A teleportation device constructed by the civilization was discovered in 2115 at Site 1 and led the UAC to attempt to reverse engineer the device, in spite of encoded warnings against this left by the ancient civilization. This device was what destroyed the civilization, as it turned out that the technology inadvertently opened a portal to Hell. After building the Soul Cube through the sacrifice of most of their population and using it to seal the portal, the few survivors left the planet and are implied to have went to Earth, where they became ancestors of modern man.

Film
In the Doom movie, Mars is featured as the initial setting of the film. A distress signal is received from the UAC's Olduvai Research Facility on the surface of Mars, and the Rapid Response Tactical Squad is called into action to investigate. It is discovered that scientists at the facility were performing classified research on a retrovirus (implied to have been genetically engineered by ancient Martians), combining it with the human genome to find that it could either give a person superhuman strength and agility, or transform them into a horrible monster, depending on that person's personality and moral qualities.

The Olduvai facility is linked with Earth via a teleportation device known as the Ark, which enables instantaneous transport between there and the Ark facility on Earth at Area 51. Because of this, however, containment of the retrovirus to Mars proves impractical - creatures escape through the portal and begin infecting the base back on Earth. John Grimm (Reaper) and the rest of his surviving squad must return to Earth to defend it.

Olduvai is named for the, a location in Africa where many early fossils have been discovered. This is likely a nod both to Doom 3's backstory in which the ancient Martians were ancestors of modern man, and an allusion to the film , which places its opening scene in that location.