Using Fleep, Mario will have to find a thunder mark in Flipside and flip it to reveal a secret passage. Mario must flip to 3D, enter, go to the door at the end, and enter it to find himself in Mirror Hall. The Mirror Hall, when first approached, has eight blocks and a mirror in the center.
Mario must hit the blocks in order from right to left until he has hit all of them. After that, the mirror will break. Mario must flip to 3D, enter the mirror, and find himself in the Flopside section of the Mirror Hall. At the end of the Flopside part of Mirror Hall is a door going to the town of Flopside.
Passing the Mirror Hall is the only free way of getting from Flipside to Flopside. However, the player can find Welderberg in two parts of Flipside who, if paid 300 coins, will build a pair of arguably fast Warp Pipes between the interdimensional cities.