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.
The entrance to the enigmatic glitchy World is at the ending of the World 1-2, between the first and second green pipes.
How to access
This is a glitch world. In the NES, Virtual Console, and Game Boy Advance versions, this is a never-ending underwater world. When you reach the end of this "level", you are taken to the start of the level. There are no powerups and the only way to end the level is to die or run out of time. When you die, you are taken to the start or middle of -1. In the Famicom Disk System version, this is a three-level world with some strange quirks. The game ends after the third world as though you completed the game at world 8. The Minus World is only available in the NES, Famicom Disk System, Virtual Console, and Game Boy Advance versions. In Super Mario All-Stars, you can still go through the wall but the pipes take you where they normally would. In Super Mario Bros. Deluxe, you can't even go through the wall.
To access this glitch, become either Super Mario or Fire Mario. Go to the end of World 1-2 and stand on top of the pipe. Go all the way to the right, and then one square left. Knock the brick above you, and the one to the left of it, out. There should now be the wall with one brick attached and a gap.
Walk to the edge of the pipe without falling off. Now for the tricky part: crouch, then while crouching, jump and hold right so the brick attached to the wall pushes you down in the air slightly. When done right, you will go through the wall. Do not move; you will slide through the wall automatically. Once you are through the wall, enter the first pipe (beware the piranha plant!). Do not walk all the way to the right or the area reverts to the normal warp area when "Welcome to Warp Zone!" appears (the middle pipe takes you to world 5, and the leftmost pipe also takes you to the Minus World). This was removed in Vs. Super Mario Bros.
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.
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.