From the Super Mario Wiki
Spin Drills are items that appear in Super Mario Galaxy 2. Mario and his brother can pick up a Spin Drill and drill directly through planets and dirt. When standing on a spherical planet, the player can use the Drill to go through the surface to the other side by shaking the Wii Remote. The Spin Drill can also uncover hidden inside areas of planets that are otherwise unaccessible. It cannot drill through hard/metallic/un-drillable surfaces. If Mario gets hit, touches a Launch Star or goes into a Warp Pipe while holding the Spin Drill, it will disappear. It plays a major role in the Spin-Dig Galaxy, but it re-appears in later galaxies such as the Supermassive Galaxy, where it is used to defeat three Big Koopa Troopas, the Flipsville Galaxy during the Flipsville's New Digs and Purple Coin Spin Speed Run mission, the Cosmic Cove Galaxy at the last planet of the "Exploring the Cosmic Cavern" level, and Bowser's Galaxy Generator. The Spin Drill can also be used to defeat enemies.
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