The secret world in Super Mario Bros. from which there is no escape.

To enter World -1 (or the Minus World), go to World 1-2. Near the end of the level, you can go over the top and warp to levels 2, 3, or 4, or you can go to the pipe that leads to the flagpole and World 1-3. Go to that pipe instead, and get on top of it. There are three or four blocks above the pipe that you can break if you are big. Break two near the end of the pipe, leaving one sticking out of the wall. Do a backwards ducking jump against that block. After a few tries, you should get stuck in the wall. Walk to the right and duck into the first pipe you come to. You will be sent to World -1.

There's no way out of World -1!

It's an endless underwater level that does nothing but tire out your swimming finger. It's just an interesting curiousity, and nothing more. Use up your lives, start over, and never go there again, except to show your friends how cool you are. Then you can show them the Woop Woop Trick and make them bow before you.