World 9 (Super Mario Bros.: The Lost Levels)

World 9, also known as Fantasy World or "9 World", is the first secret world in Super Mario Bros.: The Lost Levels, unlocked when the player defeats Bowser without using any Warp Zones to advance during the game. This world has three levels that take place underwater. According to an interview given in Family Computer Magazine, Shigeru Miyamoto added World 9 due to the interest in the glitch, Minus World-esque "World-9" levels in the Japanese version of the original Super Mario Bros.

The mechanics of World 9 differ depending on the version of the game. In the original release, the player is given one life to get through World 9; if the player dies at any point in this world, they automatically receive a game over (although the life counter can still be raised by collecting 100 coins), while in the Super Mario All-Stars version, the player retains all the lives they had upon completing World 8-4. Additionally, in the original version, the four levels of World 9 loop endlessly each time World 9-4 is cleared until the player receives a Game Over, while in the All-Stars version, clearing World 9 allows the player to advance on to World A. World 9 is unused in the Super Mario Bros. Deluxe version, along with Worlds A, B, C and D.

The first level of world 9 starts out with the player having to jump up a staircase of blocks; on the other side of the staircase resides a Warp Pipe, (hosting a Piranha Plant), that will take the player to the underwater section of the level. Once in the underwater section, the player will encounter a few Koopa Paratroopas and a Bill Blaster amidst a row of ? Blocks. As the player progresses, he or she will have to swim through a few Warp Pipes to continue. Past this point, several more Koopas and a Lakitu will be encountered. The player must either defeat them with fireballs or swim away from them to progress in the level. Several more enemies (such as Goombas, Hammer Bros., and Bloopers will be found towards the end of the level. After an encounter with two Buzzy Beetles and another Koopa Paratroopa, the player will pass up another staircase. Past here is the Flagpole; once touched, the level ends.

To surpass this level, the player must swim through many mazes of Warp Pipes and Piranha Plants while being chased by a Lakitu. Very few portions of this level have a sea bed; as such, the player must swim almost nonstop to avoid falling into the ocean's abyss and losing a life. Once the player reaches the three-fourths mark of the level, the Lakitu will go away (if not defeated already). The player will find a small section of platforms hosting several coins. After this point resides a staircase and the end-of-level Flagpole.

Just as the previous one was, this level is relatively short. In this level, the player is once again on dry land. Despite being a normal overworld level, the level is in the castle style and plays the castle BGM. The player must run through various mazes of bricks and lava pits to get to Bowser. There is a Coin Heaven located in this level, via the access of a Brick Block Vine and Warp Pipe (both Coin Heavens are the same and drops players off in the same area). The player is not required to fight a false Bowser to complete the level; rather, he or she can run along a brick path hovering high above the false Bowser. The player can then access the Flagpole past here, thus ending the level.

The final level in World 9 is once again located underwater. The player must swim through mazes of bricks to surpass this level; however, many enemies (including a Podoboo) block many of the narrow paths that the player must take in order to continue throughout the level. Once the player makes it through the massive cluster of enemies and blocks, there is an empty space located just before a staircase. After the staircase, a Flagpole can be found. Once the player touches the Flagpole, the level ends. Once this world is completed, World A is unlocked. Some of the blocks in this level are arranged in Japanese; they read "アリガトウ!" Arigatou!, meaning "Thank You!" in Japanese.

Level maps
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Trivia

 * This is one of only two worlds in the Mario side scrolling games to not include a castle level. The other is World 9 in New Super Mario Bros Wii. Level 9-3 does, however, have a similar theme to the castle levels.