Moon

In Paper Mario: The Thousand-Year Door, this is where the X-Naut Fortress is located. It can only be accessed via a cannon at Fahr Outpost or the Teleporter located in Rogueport Sewers. According to Goombella, a Goomba landed on the Moon in '69.

Many confuse this with the tiny moon in World 5-8 of Super Mario World 2: Yoshi's Island.