Many of today's PC games allow you to make your own levels, characters etc like Half-Life, Quake and Unreal but for older games stuff like that was left to hacking. Well, some guy has taken Super Mario World and hacked in a bunch of new levels. Of course the only possibly legal way to play is to get the modifications from him, take your own cartridge and some out download it into your computer, apply the changes, and then play.