…for the original doom, episode 1 (knee-deep in the dead) was set on phobos; episode 2 (on the shores of hell) was set on deimos hovering in the skies over hell; and episode 3 (inferno) was set down on the surface of hell itself…episode 4 (thy flesh consumed) was set back on earth after the invasion of hell, but it was an add-on released a couple years after the original game…