Reaching the Minus World
The Minus World is actually a glitch, so reaching it is a bit tricky. To get there, you must be on Stage 1-2 and you must be either Super or Fire Mario so that you can crouch. When you reach the pipe that leads you out of the underground and to the end of the level, stop. Climb on top of the pipe. Break the second and third blocks to the left of the pipe, breaking the second first and then the third. Leave the block closest to the pipe unbroken. If you break it by mistake, wait for the timer to run out and try again. Then walk to the edge of the pipe and duck. Jump while ducking and try to move into the side of the remaining block next to the pipe. If this works properly, Mario will then be pulled into the wall and travel to the right until he drops into the Warp Zone. Be aware that if you do anything to leave the wall once you have been pulled into it, you will not be able to activate the glitch properly. Enter the first pipe to be taken to the Minus World. The pipe on the left will take Mario to Minus World, the center pipe will take Mario to World 5-1. The pipe on the right will still take Mario to World 2-1 as with the regular warp zone.
Minus World is actually identical to World 2-2 with three key differences.
First, when loading up the world, Mario will be shown to be entering World -1. Minus world is actually World 36, however, since the character 36 isn't intended to be displayed, a blank space displayed instead, leaving the dash to separate the level and the world. This glitch led to gamers coining the term "minus world."
The pipe at the end of the level in 2-2 will take Mario or Luigi to the level-ending flagpole. In minus world, the pipe simply loops back around to the beginning of the level.
The other difference is in the checkpoint system. In Minus World, the checkpoint places Mario or Luigi inside a bottomless pit. If the player does not immediately tap A to swim, the character will be sucked into the abyss. In World 2-2, the checkpoint comes immediately after this pit.
It has been theorized that Minus world is a test level used in the development of the game. Using special software, it has been revealed that there are actually 5 minus worlds present on the cartridge. These Worlds are very different, and one of them is actually a blank screen.
On the Famicom Disk System version of Super Mario Bros, Minus World is accessed in the same manner, but is actually identical World 1-3 but with underwater gameplay. The player can access further minus worlds in this version by swimming and touching the flagpole lower than halfway down. World -2 is identical to world 7-3. If World -2 is completed, the player goes to world -3, identical to world 4-4. If World -3 is completed, the player is taken back to the title screen as if he has completed the game.
The Minus World can even be accessed in the special mirrored version of the game, Super Luigi Bros. as part of NES Remix 2. There, players can then play a left-scrolling version of the Minus World.