World 9 (Super Mario Bros.: The Lost Levels)
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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 and underwater palettes, causing parts of the landscape to appear out of place and very discolored (intentionally matching the Minus World-esque glitch effect mentioned above). In the SNES version, overworld tiles are still used, but with more accurate palettes.
In Volume 52 for Nintendo Power, a contest was held where the player needed to get to World 9 in Super Mario Bros. The Lost Levels, and supply photographic proof that they did so. Anyone who succeeded in doing so would be supplied with an iron-on patchMedia:World-9-patch.png commemorating the occasion.
All secret worlds are inaccessible in the Super Mario Bros. Deluxe port, World 9 included.
The Lost Levels
As noted above, the Game Over for World 9 in the original Disk System version was unique compared to the other Worlds. Specifically, the world's game over lacks the options for continuing the current level or retrying the game. Instead, in its place, there is a message attributed to Mario as well as the staff for the game telling the player that they are a "super player", and that they hope they'll see the player again.
In the SNES version, it just uses the standard game over screen.