From the Super Mario Wiki
Painted Swampland or Soda Jungle-4, is the fourth course of Soda Jungle in New Super Mario Bros. U. It is located in the haunted sub-area of Soda Jungle. Most notably, this course is an outside ghost course with the design and art style inspired by Vincent van Gogh's The Starry Night.
The bottom of the course is filled with poison, and the most prevalent enemies are Boos and Circling Boo Buddies. Most of the platforms are pipes, some of which are slanted and sink into the toxic water when the player steps on them. Completing the course normally makes a path that leads back to the first point after Which-Way Labyrinth, but completing the level with its secret exit reveals the true path through Soda Jungle. Also, near and in the end of this stage, there is Bowser with no pupils in the background.
Unlike most secret exits in the game, this course's secret exit is required to continue through Soda Jungle. After a series of red pipes, there's a green slanted one leading to a wall. The wall contains a Warp Pipe that leads to the course's secret exit.
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