The minus world in Super Mario Bros. 1. If you have never found it, Reach the exit pipe in level 2, and break the 2 next to last bricks. Face left, and hit the back of Mario's head against the last brick WITHOUT BREAKING IT!!! If you do it right, you will be pulled through the wall into the warp zone area, but it will just be three pipes, not a warp zone. Go into one of them, I thunk it's the third one, but I could be wrong. You will then be in the first water world, labeled -2. Then, you will play the level for all eternity. Enjoy!