Minus World

The Minus World is a glitch found in Super Mario Bros. It is an underwater level that has no end, meaning that once there, Mario or Luigi is doomed to die by either running out of time or being killed by enemies. The level got its name because, when in the Minus World, the top screen says that you are in World 0-1, effectively World -1.

To get to the Minus World, you must go to World 1-2 and stand on the pipe that leads to the flagpole at the end of the level. Break two blocks above the pipe, but leave the one to the far right. Now, have Mario move to the far left of the pipe, facing left. Jump into the air, and press right while in midair. When you come back down, you should be in a warp room if you did everything correctly. Once the worlds in the room appear, take the pipe to the far left, and you'll be in the Minus World.