The Minus World, or World -1, is a famous glitch in the original Super Mario Bros. on the NES. It's not an intended feature but a fascinating Easter egg. To access it, go to World 1-2. At the very end of the level, just before the exit pipe, there's a brick block and a pipe. Break the brick block, then stand on the pipe. While standing on the pipe, jump and move left, trying to clip Mario's head into the wall. If done correctly, Mario will pass through the wall and enter a pipe that leads to World -1. This world is an endless underwater level, and there's no way to exit it normally, forcing you to reset the game. It's a fun curiosity but not a path to victory.
