Dreadnought's Garbage Dump
From the Super Mario Wiki
On the underside of the UFO the player starts on is a Warp Pipe that will take the player to a platform with a Gearmo and piles of trash. Talking to the Gearmo and accepting his task will begin the challenge of destroying all the garbage piles with Bob-ombs within a time limit of thirty seconds. Completing the challenge will have the Gearmo reward the player with the Power Star.
The Coin Generators on the ground serve as good targets for where to throw Bob-ombs.
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