World Mushroom (New Super Mario Bros. 2)
From the Super Mario Wiki
World Mushroom (or Mushroom World) is the first special world and the seventh world appearing in New Super Mario Bros. 2. This world has a generic design which is similar to that of World Flower and World Star. The background color of this world is pink. It is an alternative world to World 2 and thus it is optional. This world can be accessed by completing World 1-. There are three Toad Houses in this world, one of each of red, green, and yellow varieties. Notably, it is sometimes considered to be the seventh world of the game, as the numbers needed to unlock the Rainbow Course are two "7"s, but it is also sometimes considered to be preceding World 3.
There are nine levels in this world and, unlike normal worlds, does not have a tower level. This world has a total of eleven exits. There is also only three normal levels on the main path in this world, contradictory to the five-to-six levels found in normal worlds. Levels in this world (other than World Mushroom-Rainbow and World Mushroom-Warp Cannon) can be based on any theme. There is also a Ghost House in this world. There are also two secret levels which the players have to find the secret exits for. The world ends, as always, with the castle level, where Larry Koopa, the boss of this world, is waiting, unless the player takes the path to the Warp Cannon, which will lead to World Flower when completed, allowing the player to skip an additional two worlds, World 3 and World 4.
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