The Minus World is a glitch level found in Super Mario Bros. and is arguably regarded as the most famous glitch in video game history. It is an underwater level (exactly like World 7-2 in geography and enemy locations) that cannot be finished (the pipe at the end is glitched and takes the player back to the start), meaning that once there, Mario or Luigi is doomed to lose all of his lives by either running out of time or being defeated by enemies. The level got its name from fans because, when in the Minus World, the top screen says that Mario is in World -1. This is because of a glitch in the panels displaying the numbers. The Minus World is actually World 36, but the game is displaying the blank graphic for the number 36, leading to -1.
The player can get to the Minus World by going to World 1-2 and standing on the pipe that leads to the Flagpole at the end of the level. Then, they should move to the far left end of the pipe, then duck while facing left. While ducking, the player should jump. While in the air, the player should move right, so that when he's about to come back down, he should be just under the ceiling next within this moment. It may take the player several tries, for Mario must land in an exact spot on the pipe in order to travel through it. If done correctly, he will automatically move through the pipe (without the screen moving) and then through the wall and into the warp room. If not even 1 pixel of the pipe is showing, Mario will be trapped until the timer runs out. 
Another method of reaching the Minus World involves Mario breaking two blocks on the ceiling, leaving the one on the far right. He should move to the far left end of the pipe and jump to the right. He doesn't have to duck as he can try to hit the bottom-left portion of the block (without actually destroying the block). This should glitch the game, allowing Mario to get through the pipe and wall, but doing it this way is potentially more difficult and time-intensive.
If everything is done properly, Mario will see the three pipes in front of him from the Warp Zone. If he enters the pipe to the far left or far right, he will find himself in the Minus World. If he goes in the pipe in the middle, it will bring him to World 5-1. If the player makes the brick wall on the far right visible, however, the glitch will end. The reason the middle pipe leads to 5-1 is because the game is loading data from the 4-2 over-the-top warp zone (not the one above the vine) that has only one pipe to 5-1. The area of the other pipes is filled in with Tile 36 (the blank tile causing the -1 display), causing it to think it leads to World 36-1, that is, the Minus World.
Famicom Disk System
In the Famicom Disk System version of Super Mario Bros., the Minus World is accessed the same way, but is very different in design. World -1 is just like World 1-3, but with underwater gameplay (despite lacking actual visual water) and featuring bizarre elements such as multiple floating Princess Toadstools and a floating, headless Bowser A misplaced Hammer Brother, as well as an overall glitchy palette for several objects.
This marks the only instance of an underwater flagpole (without the flag), which can easily stop the game from progressing if touched too high or initiating a fireworks display; if reached normally halfway down or lower, this flagpole will actually take Mario to World -2, a level identical to World 7-3. There is finally a World -3 after this level; -3 is a version of World 4-4 that lacks its Bowser Impostor and maze elements, is colored underground, and is filled with flying Bloopers that can be stomped for 1000 points. This behavior is otherwise unused, though nearly identical flying Bloopers would be featured in Super Mario Bros.: The Lost Levels.
Beating World -3 takes the player to the title screen. If the player starts the game again after going through the Minus World, it will replace all Goombas with Buzzy Beetles and will be like Hard Mode.
Vs. Super Mario Bros.
In the arcade version of Super Mario Bros., which is harder than the original home console version, some bricks located above the end pipe are removed to prevent the trick from being performed.
However, it is not impossible to access. It can be accessed by clipping through the bricks under the pipe.
All Night Nippon Super Mario Bros. / Super Mario Bros.: The Lost Levels
The Minus World glitch has been "removed" from remakes of Super Mario Bros. In Super Mario All-Stars, if the trick is performed, the Warp Pipe will still take Mario to 4-1 as if the pipes were reached by walking along the ceiling. The Warp Zone text will read "5", with nothing over the other two pipes if the player is quick enough jumping down the middle pipe. However, it will still take the player to 3-1 for some reason. In Super Mario Bros. Deluxe, the ability to perform the trick has been removed entirely.
If one uses a gaming tool that can set the level that Mario is on, the Minus World goes all the way to -9. After -9 it changes to -A, later to -B, -C, and so on. After the letters, it changes to random bits of the terrain of all the levels. There are 215 of these bits. These levels are the result of level layouts being loaded with the wrong tilesets, enemy layouts, and properties, and are very strange. Specifically, World -2 is a version of World 3-4 with water graphics; World -3 is a version of World 3-4 with overworld graphics and World -4 and World -5 are just a blank screen. World -6 is a glitched out level which appears in a night setting. Some gray bricks may be found on the edge of it and a white Goomba will fall into the pit. The entire level is a pit, so Mario will fall down, to his death. There's a glitch which allows the player to complete the level by repeatedly pressing the pause button, which will lead Mario to a copy of World 3-3. After passing it, the game takes the player to -7, which starts off using the intro for 1-2, 2-2, 4-2 and 7-2, although the game blacks out after Mario reaches the pipe. -8 is a weird castle level that you cannot move in, and -9 is a black screen. All the levels after -4 can only be played by using cheats or hacking.
As shown in some editor utilities, the first Warp Zone that can take the player to the Minus World initially loads the data for the second Warp Zone, which normally only has one warp pipe in the center, until the "Welcome to Warp Zone!" message appears. The two empty slots on the left and right sides would read "36" if a graphic existed for that number. If two additional pipes are added in the appropriate places in this Warp Zone, they send Mario to the Minus World without executing the glitch. This explains why the second pipe, if the player performs the glitch, takes the player to World 5 instead of World 3.
Names in Other Languages