From the Super Mario Wiki
Icicle Caverns, or Frosted Glacier-5, is the fifth level of Frosted Glacier in New Super Mario Bros. U. It is unlocked via completion of Prickly Goombas!. Its own completion opens the path to Swaying Ghost House. It is also the only level with Ice Bros.
Most of this level takes place in an underground cave that is made almost entirely out of ice. A common hazard in this level are the icicles covering the ceiling. As with Freezing-Rain Tower, the icicles vary in size and can fall onto the player. Once in a while, there are a few icicles that do not fall on the player when the player passes them.
A P-Acorn or a Balloon Baby Yoshi is required for this one. At the beginning, just before the pipe, when the giant icicle falls, a character with a P-Acorn or Balloon Baby Yoshi can fly or float to the ceiling and find a secret hole. Mario must go right to find some coins. If Mario goes more right, then he could get to the flagpole area. This is also the quickest way to obtain the third Star Coin.
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