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

World 9, also presented in in-game text as Fantasy World and “9 World”, is the first secret world and the ninth world in Super Mario Bros.: The Lost Levels, unlocked when the player defeats Bowser while clearing all 32 of the game's standard levels in that playthrough. Usually this entails using no Warp Zones, though it is possible to use the backwards Warp Zone in 3-1 to return to 1-1, then warp from 1-2 forward to 3-1 again, or use the backwards Warp Zone in 8-1 to return to 5-1, then warp from 5-2 forward to 8-1 again and still unlock World 9. Unlike all other worlds in the game, this world has three levels that take place underwater, as well as a castle that is not the last level of the world. According to an interview given in Family Computer Magazine, Shigeru Miyamoto added World 9 to the game due to interest in a glitch exclusive to the Japanese version of the original Super Mario Bros., where using said glitch to access that game's nonexistent World 9 would load an overworld level with the underwater palette, creating a Minus World-esque effect.

The mechanics of World 9 differ depending on the version of the game. In the original release, the player is given only one life to get through World 9 (though they keep their score, coins, and power-up status from World 8-4); 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 or at the flagpole when the right digits match). The game over screen for this world, like the opening screen, features a unique message; as a result, the usual options are not displayed and the player effectively cannot choose to Continue. If the player clears World 9-4, the game goes back to World 9-1 and the four levels of World 9 loop endlessly until the player receives a Game Over. In the Super Mario All-Stars remake, the player retains all the lives they had upon completing World 8-4, and clearing World 9 advances the player onward to World A.

Aesthetically, Worlds 9-1 and 9-2 in the original FDS version appear as flooded overworld levels with some underwater tiles, causing parts of the landscape to appear out of place (intentionally matching the Minus World-esque glitch effect mentioned above). However, in the SNES version, these levels appear as regular underwater levels.

All secret worlds are inaccessible in the Super Mario Bros. Deluxe port, World 9 included.

The Lost Levels
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All-Stars
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Trivia

 * This is one of only two playable worlds in the Mario side-scrolling games to not include a true castle level. The other is World 9 in New Super Mario Bros. Wii. However, World 9-3 does have a similar theme to the castle levels, though it takes place outside.
 * Consequently, World 9-3 is the only level in the game to be an overworld level with castle music.
 * World 9-3's Coin Heaven is the only one in the game without a scrolling platform, leaving the player free to collect all the coins at their leisure.