The Minus World is a world that you can get via World 1-2 by using a glitch where you can slide through blocks, (search "how to get to the negative world") follow those instructions and, welcome to World -1. There is no finish except if you add a flag with a level editor. World -2 same principle. World -3 same principle. However World -4 is a black gameover screen. Originally the world number is 36, however Super Mario Bros. isn't programmed to display level numbers which value is more than 9.