Those levels, however, are messed up.
They're broken, contain enemies that have no place within them (a Goomba underwater?!), and look completely bananas. They're the bastard children of Nintendo's most revered game, hidden away in the annals of each cartridge's aging chips. These are levels Nintendo never intended anyone to see, and the company has never really spoken about them as a result.
But intrepid Nintendo enthusiasts have cracked the crazy secret process for unlocking them.
Here's how to do it.