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 from fans because, when in the Minus World, the top screen says that Mario is in World 0-1, effectively World -1 (or Minus World).

Mario 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. Mario should break two blocks, leaving the one on the far right. Now, Mario should move to the far left end of the pipe, facing left, and jump into the air. While in the air, Mario should move right, so that when he comes back down, he moves through the wall and appears in the warp room. If everything was done correctly, Mario should see three pipes in front of him. If Mario enters the pipe to the far left, he will find himself in the Minus World.