Deimos

Deimos is the smaller and outermost of the two moons of the planet Mars, the other being Phobos. It is the scene of the second Doom episode, "The Shores of Hell". Deimos is named after the Greek god of dread and terror, and a son of Ares.

Doom Classic
According to the Doom manual, Deimos is used to store nuclear waste, and is also the home of teleportation experimentation with Phobos. Sometime before the game however, an experiment goes wrong, Deimos vanishes, and the Phobos base is attacked by an unknown force, with the player's squad being sent up to investigate.

By the time the Lone Marine crossed over to Deimos, he found the human personnel there slain or transformed into zombies, and the complex was now under the command of a Cyberdemon and its minions, who were gradually transforming the advanced facilities built there by humans into a grim bastion of great evil. However, the Marine was able to battle his way to the Cyberdemon's lair at the Tower of Babel, and destroyed its infernal master, avenging the loss of human life on the moon station.

Doom 64
Deimos could be indirectly mentioned in the manual for Doom 64. The choice of words the manual uses could imply that Deimos has somehow returned to its orbit around Mars (it mentions 'installations' plural presumably Phobos and Deimos, Our Hero believed he had cleaned up previously during his 'first mission' during the classified "Doom episodes"). However, it could also be referencing the base from Final Doom's TNT: Evilution, which takes place on an unspecified moon of Jupiter.

The official website for the game and the short story article from Nintendo Power states that Doom 64 takes place on Phobos.

Doom RPG
Deimos isn't shown physically in the fly-in in the intro, however its orbiting projection is shown under Phobos.

Doom (2016)/Eternal
In new Doom series, it is set in a parallel universe to that of the original Doom. In this universe, Deimos never was absorbed into Hell. In DOOM, Deimos and Phobos are used as mining stations, although it's never stated, or implied that any outbreaks, or military operations occurred there.


 * The Gauss accelerator design has been used by the UAC as the basis for numerous industrial projects over the years - such as the Argent Tower itself, and the Ore Diggers on Deimos and Phobos.

In Doom Eternal, once you reach the Arc level, the background of the main menu/game selection screen shows Mars, Phobos, and what looks to be the moon of Deimos.

The hologram of Mars which appears on the Fortress of Doom, shows the existence of both Phobos and Deimos orbiting the planet.

Behind the Scenes
In Doom, Deimos is represented as having older style bases (barring the presence of Hell corruption) composed of brick and cement walls. The shape of Deimos levels are notably square compared to the abstract-design of the Phobos, and Inferno levels. Deimos is also represented as having large brown mountains capped with small amounts of snow, despite the intermission screen showing a grey landscape, similar to how Phobos is shown having large grey mountains covered in vegetation, but a brown cratered landscape in the intermission screen, and in levels.

In reality, Deimos is much smaller than Phobos, only having a radius of 6.2 kilometers, possess little gravity, and no atmosphere (even if it had an atmosphere, it would drift off without some form of artificial gravity technology).