From the Super Mario Wiki, the Mario encyclopedia
Tilted Tunnel, or Acorn Plains-2, is the second course of Acorn Plains in New Super Mario Bros. U. It is an underground course that features 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 level begins next to a ? Block containing a power-up, as well as a Warp Pipe leading underground. Some tilted Warp Pipes containing Piranha Plants can be found, along with Goombas. The cave splits into two paths, with the bottom path containing coins and a Koopa Troopa. More Piranha Plants and Goombas are encountered in the top path, which leads to a group of shifting crystals. A section with more crystals follows, along with enemies such as Goombas and Piranha Plants. A sloped section with a row of Goombas is found, allowing the player to gain 1-Ups. Tilted Warp Pipes containing Piranha Plants are found, along with the Checkpoint Flag. More Piranha Plants in Warp Pipes are encountered, along with a Warp Pipe that leads to a bonus area full of Buzzy Beetles and a POW Block, which can be used to make a shower of coins fall down. More Koopa Troopas and Piranha Plants are encountered. A shifting crystal with a Red Ring above it is found, along with a Koopa Troopa and a Goomba. More crystals and Piranha Plants are found in the section leading to a Warp Pipe that takes the player to the Goal Pole.
Alternatively, the player can use a P-Acorn to fly up above the wall before the pipe leading underground, which will lead to the end of the level.
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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