Mars is one of the locations in Yume Nikki, and is accessed by going into the Spaceship, returning to the entrance room, sleeping in the bed until the room is no longer visible. Wake up when the alarm goes off, and after a few moments, the Spaceship will crash. It is a dead end in terms of gameplay, requiring the player to either wake up or use the Medamaude.
Mars is a straight-line area, with no branching paths. Going to the left brings you across the barren planet until a mountain appears (in reality the ground being turned on an angle) and brings you to the top, in which a transformation into a Midget is necessary to get into the small hole with steam coming out to see Mars-san. If you use the Cat effect small objects (presumably other Spaceships looking for the crashed one) will warp in on the background.