Massive Mushroom Maze

Massive Mushroom Maze is a secret area in Toad Town, found in a basement of a house. It is a large maze made of mushrooms with several Toads throughout it. Upon reaching the center of the maze, the player will find a hidden star. This area's star does not contribute to the Toad star counter.

Theorizing
It is possible this level is the unused 3KINOP level. 3KINOP's map is a large maze. Toad's name in Japanese is Kinopio, and seeing as 3KINOP is a shortened version of Kinopio, and Massive Mushroom Maze is themed around Toads and mushrooms, and that both levels are mazes it is very likely that this level is 3KINOP.