From the Super Mario Wiki, the Mario encyclopedia
Crooked Cavern, or Acorn Plains-2, is the second course of Acorn Plains, and subsequently, the second course of New Super Luigi U. It is unlocked by completing Waddlewing Warning!, and its own completion unlocks Flame-Gear Tower and the first Toad House.
The first area is a small above-ground field where three Warp Pipes are found. The left and right ones are red and contain Piranha Plants, while the middle one is green and leads to an underground section. The second area begins on a moving crystal platform, along with Warp Pipes containing Piranha Plants. More crystal platforms are found, along with more Piranha Plants. Some crystal platforms with Koopa Troopas on them are also found. More crystal platforms follow, with Brick Block platforms on the edges of them. Another crystal platform is found, and has more Koopa Troopas and Piranha Plants on it. A Warp Pipe is found above a platform near a large pit. The Warp Pipe leads above ground, where the Goal Pole is located.
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