I found this weird video of a lost level in Super Mario Bros. The game is supposed to end on world 8, but according to the comments in the video and google translate, you can access world 9 with some arcane methods like turning of your NES quickly, re-inserting the cartridge, resetting and some variations of that. It's also very likely that someone just made this levels up in an editor. Whether it's true or not I like the colors and design of the levels. Here's the video:
According to the comments in the video (translated from japanese) you can access the levels like this way:
Indeed, I think it was like this.
1) Mario startup screen, and run out of power, a cassette of a suddenPull
2) Put a tray of tennis, a positive screen by pressing the reset button a few times I will always play tennis
3) turning off the power supply, replace the cassette Mario
4) to start the game properly reset button on the screen, and seen They come out at random surface
If you finish the game without warping in Super Mario Bros: The Lost Levels (Super Mario Bros 2 in Japan) you get to a similar set of aquatic levels after world 8, but they seem to be different to the levels found in this video.