Doop Doop Dunes Grotto
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While traveling through Doop Doop Dunes, Mario and co get ambushed by the Big Pokey and Tower Power Pokey, who knock them off the bridge into the Doop Doop Dunes Grotto. Realizing that the two Pokeys are on top of a high ledge that cannot be reached from the ground, the trio explore the grotto and make their way out to the Toad Village, where they ask the Paper Toads for an item to "get to high places".
Getting a trampoline after completing a few more Toad missions, the trio return to the Grotto, and use it to reach the Pokey Duo, who they battle in a boss fight.
After this is done, they never have to return to the grotto for the rest of the game.