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 (There is one exception to this rule, if the player clears Worlds 1 and 2, accidentally uses the backwards warp in 3-1 to 1-1, and then uses the warp in 1-2 to return to World 3, it will not be counted against the player). 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.

World 9-1 and 9-2 in the NES version appears as flooded overworld levels with some underwater tiles. However, in SNES version these levels appears as regular underwater levels.

The Lost Levels
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All-Stars
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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.