This course, based on World 5 takes place in a snowy forest, with floors made of ice blocks containing Munchers and pipes containing Venus Fire Traps. This volcanic course is based on World 8 and contains numerous platforms which emerge from and descend into lava. This is an icy course based on World 3, where the player must get to the top of the Icy Mountain while avoiding Cooligans, Fuzzies, and Big Fuzzies. This course takes place over the desert landscape of World 2 and is auto-scrolling. It features Missile Bills and King Bills. This course has the background of World 6. The player must get to the goal while avoiding a Porcupuffer and many Koopa Troopas. This stage takes place in the beach setting from World 4.

There are spinning rectangle platforms throughout the level. The first level of World 9 is sky-themed and has the athletic background from World 1. Peach alludes to this world in the first ending, where she asks Mario if she ever told him about the secret world while departing back for the Mushroom Kingdom. Unlike in previous worlds, the levels can be played in any order, and no Enemy Courses, fortresses, or castles (or levels based on them) appear in this world. World 9 has a total of 24 Star Coins, which are needed if the player wants to get all five stars on their profile on the file select screen. The rainbow theme is similar to the Rainbow Road courses in the Mario Kart series, with the world map music even being an arrangement of Rainbow Road's music from Mario Kart 64.

The Super Guide will also not appear in World 9, even if the player dies eight times in the same level. The levels in World 9 are among the most difficult in the game, and none of them have midway points in them, which adds to the world's challenge (like the Special Zone from Super Mario World). Each level features the background and a music track from one of the previous eight worlds. For example, collecting all Star Coins in World 1 will unlock World 9-1.

Each of the eight levels in this world is unlocked by collecting all of the Star Coins in the world of the corresponding level number. It is a special world unlocked by defeating Bowser in World 8. World 9 is the final world from New Super Mario Bros. “Oh, Mario, did I ever tell you about that secret world?” - Princess Peach, New Super Mario Bros.

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