Mars

Mars is the fourth planet from the sun in the Solar System. The story to Doom takes place on Phobos and Deimos, the two moons of mars, while Doom 3 is set on Mars itself, 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 up their 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.