Phobos

Phobos is the larger and innermost of the two moons of the planet Mars, the second being Deimos. It is the scene of the first Doom episode, Knee-Deep in the Dead. "Phobos" is Greek for "fright".

In Doom, Phobos is depicted with Earth-like gravity, a thick atmosphere, and having tall, seemingly vegetation-covered mountains. In reality, Phobos is a rock 20 kilometers in diameter with gravity less than a thousandth of that on Earth, and no atmosphere (even if an atmosphere could be generated artificially, the gravity would be insufficient to hold it in place). Phobos' gravity is so weak that a human being could escape it by jumping. In order to be more plausible, Doom 3 moved the plot to Mars.