Cheep Chomp Chase
From the Super Mario Wiki
Cheep Chomp Chase or Acorn Plains- is the secret level of Acorn Plains in New Super Luigi U. It can be accessed after reaching the secret exit of Crooked Cavern. The completion of this level unlocks Giant Swing-Along.
The level starts in a pond with Cheep Cheeps, many yellow pipes, and one red Warp Pipe that leads to the underwater area, where the Cheep Chomp attacks. After this, schools of Eep Cheeps and Deep Cheeps appear while Luigi swims near L-shaped and spinning air pipes. To the very right, there is the way to the exit. The next area contains the two final groups of spinning pipes, some other Cheep Cheeps, and the Goal Pole.
Near a yellow pipe above the Red Ring, there is a swimming 8-bit Luigi.