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

World 9 is the first secret world in Super Mario Bros.: The Lost Levels unlocked when the player defeats Bowser without using any Warp Zones during the game. This world has three levels that take place underwater.

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 Warp Pipe. The player is not required to fight Bowser to complete the level; rather, he or she can run along a brick path hovering high above 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.

Trivia

 * Some of the blocks in World 9-4 are arranged in Japanese; they read "アリガトウ!" Arigatou!, meaning "Thank You!" in Japanese.
 * World 9-3 must be the easiest level unless you want to battle Bowser.