From the Super Mario Wiki
Tilted Tunnel, or Acorn Plains-2, is the second course of Acorn Plains in New Super Mario Bros. U. It is an underground course with slanted pipes and shifting crystals. It is the first course to feature the Fire Flower and to have a secret exit. It is unlocked via completion of Acorn Plains Way, and its own completion unlocks Crushing-Cogs Tower. After this level is completed, a cutscene shows the flags on Peach's Castle being replaced by Bowser's flags.
The course is filled with slanted Warp Pipes, which Piranha Plants come out of, and shifting crystals, which move when the player gets to a specific point. Later in the course, there is a long pathway where the player can slide into Goombas to gain 1-Ups. There is also a bonus area full of Buzzy Beetles and a POW Block. The player can toss the POW Block to make a shower of coins fall down.
Near the end of the course, there is a pink crystal with a Red Ring that shifts upwards. The player can use that crystal to jump up across the ceiling on top of the course. The player can then jump off a blue crystal to reach an even higher ledge. Now Mario has to run all the way across the last part of the cave section. There is a hidden Warp Pipe at the back. Getting to this secret exit unlocks Blooper's Secret Lair.
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