|Blooper's Secret Lair|
|World location||Acorn Plains|
|Players||Mario, Luigi, Blue Toad, Yellow Toad, Mii|
|Previous level||Acorn Plains-2: Tilted Tunnel|
|Next level||Soda Jungle-1: Jungle of the Giants|
|Game(s)||New Super Mario Bros. U|
Blooper's Secret Lair is the hidden level of Acorn Plains and the first hidden level of New Super Mario Bros. U. The level is an underwater course. The level is accessible midway through Tilted Tunnel.
- Star Coin 1: Swim in between the two green pipes hanging from above and avoid the pressure that make you move away from the gap.
- Star Coin 2: Near the red pipe lies a Star Coin that looks impossible to grab. Cross by going under the red pipe but don't go too low to avoid dying.
- Star Coin 3: After finding the checkpoint, find the bubble coins leading to the pipe that isn't sticking out much on the left and go in it to reveal an area with the coin.