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. The world has been removed in Super Mario Bros. Deluxe, along with World A, B, C and D.

World 9-1
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.

World 9-2
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.

World 9-3
Just as the previous one was, this level is relatively short. In this level, the player is once again on dry land. 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.

World 9-4
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.

Trivia

 * The All-Stars version is a bit less trippy, and looks a tad more conventional. Originally, a slightly funky color pallette and smiling clouds also pervaded the landscape.
 * The All-Stars version skips the special level intro and game over screen. Also, the All-Stars version is considered a hidden part of the set of later levels; in the original, clearing World 9-4 does not progress to World A-1. Instead, the world is repeated until the player gets a game over. Mario/Luigi also starts with one life in the original incarnation, although it can still be raised with 100 Coins.
 * This is one of only 2 worlds in the Mario side scrolling games to not include a castle level. The other is World 9 in New Super Mario Bros Wii.
 * The Bowser in World 9-3 is actually an imposter, which makes sense given that he was just defeated in World 8-4. This detail is left out of the All-Stars version, as duplicate Bowsers look identical in that version.