It'd be remiss not to include the glitch that pretty much kicked off gamers' eternal hunt for secret levels in Mario games. This level above, eerily labelled '-1', Negative World or Minus World isn't actually supposed to exist, and restarts when you reach the pipe at the end, keeping Mario forever stuck in a lethal watery loop. How to unlock it: At the end of World 1-2, instead of exiting through the green pipe, jump on top of it, face left, crouch, then jump backwards to the right until Mario glitch-slides through the pipe and gets carried over to the warp zone. Jump into the first pipe to enter World -1. If you happen to own a Japanese NES, then the minus world is way more psychedelic, featuring three levels of strangely-coloured worlds, grey Bowsers, and a floating princess (foreshadowing her hovering abilities in Mario Bros. 2, perhaps?).