Flopside is a location featured only in Super Paper Mario.
Flopside is a vital location in the game, only accessible after the completion of Chapter 4. This city is the darker half of Flipside. In it, is a very similar city to Flipside in that many buildings are at the same spot, but on the opposite side. For instance, the shop, located on the left side of Flipside, is now on the far right side when it comes to Flopside. This town is first accessed in a secret chamber located in the first floor of Flipside. Here, Mario solves a riddle and proceeds into a mirror, that mirror being a small gateway into a room that leads into Flopside.
Flopside also houses the last four Heart Pillars that are used to seal the Pure Hearts. Atop Flopside Tower, only when all eight Pure Hearts are put into the pillars, will the eighth and final door into Count Bleck's realm, Castle Bleck, be opened.