From the Super Mario Wiki
Fliprus Floes, or Frosted Glacier-, is the secret course of Frosted Glacier in New Super Luigi U. It is unlocked by completing Peek-a-Boo Ghost House's secret exit, and its own completion will unlock Mount Fuzzy.
The level starts in a snowy area, near a ? Block and a Fliprus. Some platforms that bob in the water when jumped on are found, along with some icicles and a ? Block containing either a Super Mushroom or a Fire Flower, depending on the player's current form nearby. Above the ? Block is a secret area containing a 1-Up Mushroom. A Fliprus is found at the end of the area. Another Fliprus on a Brick Block containing a power-up is found above a platform bobbing in the water. A Hidden Block containing a 1-Up Mushroom is above the Brick Blocks. More platforms are found in the water, with a River Piranha Plants in between. Luigi must then cross an area with more River Piranha Plants and Flipruses. The player then reaches the Goal Pole.
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