From the Super Mario Wiki
Underground Grrrols, or Layer-Cake Desert-2, is the second course of Layer-Cake Desert in New Super Luigi U. It is unlocked by completing Spike's Tumbling Desert, and its own completion will unlock Wind-Up Tower.
The player begins the level in a desert, near a Warp Pipe. When they enter the pipe, they will be taken to an underground area where a Pokey, a Grrrol and two ? Blocks can be found. A Hidden Block containing a Yoshi Egg in it can be found above the ? Blocks. The player must then walk across multiple moving platforms where several Pokeys and Grrrols can be found. A P Switch that causes Blue Coins to appear can be found on the ceiling, near two Pokeys. Afterwards, more ? Blocks and Brick Blocks can be found above semi-solid platforms. The player must then cross more moving platforms, where they encounter Pokeys, Grrrols and a Dai Gorō. A Warp Pipe can be found above the Dai Gorō. The pipe takes the player into the desert, where a moving platform and a Goal Pole can be found.
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