World -1
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The World -1, also known as The Minus World is a hidden world which is the result of a glitch in the Warp Zone in World 1-2. Of all the secrets in the game, perhaps the most enigmatic is this famed Minus World, as well as it is one of the biggest glitches found in the game. The reason why it's called Minus World it's actually because is the product of a glitch. It's not essentially a whole World, it's basically World blank, which consists of only one standard underwater never-ending level. For obvious reasons, this World has been removed from many remakes of the game, and it's only available in the original NES game and on the Wii Virtual Console.
If you wish to access this glitched World, you need to go to World 1-2, at the very end of the still-underground part of the level, shown on the image above. Do not enter the pipe that leads back to the surface. Instead, stand on the pipe and break the second and third ceiling blocks next to the pipe. It is important to leave the block directly next to the pipe intact at all times. Then stand on the far left edge of the pipe and continue facing left. Here's the tricky part: you must make Mario jump up while crouching and to the right while still facing left. If you hit the remaining block at the proper angle without destroying it, Mario will pass through it and begin walking through the brick wall. When you enter the Warp Zone room, drop into the first pipe on the far left, and you'll be whisked away to the Minus World.
Explanation
The main reason why this glitch happens is because of a camera glitch. When Mario goes through the wall, what he's doing is going before the camera loads the level. Thus, when Mario access the Warp Zone before the camera, those Warp Pipes do not lead where they were intended in the first place. The first and the last pipe lead to the non-codified exit of World -1, while the pipe in the middle will actually lead you to World 5 without problems. And if you walk well to the right, the game will be able to load the Warp Zone properly and you'll have your Warp Zone back to normal.
