From the Super Mario Wiki
Shifting-Floor Cave, or Rock-Candy Mines-7, is the seventh course of Rock-Candy Mines in New Super Mario Bros. U. It is unlocked via completion of Screwtop Tower and its own completion unlocks a red Toad House, Roy's Conveyor Castle, and a path to the first point after Grinding-Stone Tower.
The course's main gimmick is mechanical platforms, referred to as "shifting floors", that push through the gears in the middle.
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