Hackers have been chipping away at NES game code for decades now, finding all sorts of secret levels, incomplete content and the like. Over on YouTube, user SKELUX posted the above video of his most recent discovery, which apparently took him a good six hours to crack. He calls it The Legend of Zelda's Minus World, which is basically a whole bunch of assets and stray bits of levels that were never intended to be in the actual game.
I'm not going to pretend to have a firm grasp of coding for the NES so, if you want a solid explanation of how SKELUX got this all to work, you're going to want to give that video a gander. Actually, you should probably do that anyway, as it's an interesting look behind the curtain of game development.
In short, the original Zelda map was built on a grid of 8x16 tiles, even though the chip allows for 16x16. In other words, about half of Zelda's "map" is not actually part of the game, and the game wasn't intended to function properly if players figured out a way to stray into that particular domain.
That's where SKELUX comes in, as he managed to figure out a way to finagle the code and explore those regions without the game crashing. What he found was screen after screen of areas that were clearly not intended to be explored, but it was all surprisingly stable and, to a degree, playable. Many of the enemies were glitchy or bizarre hybrids, some "characters" were bugged-out sprites that aren't recognizable as people (even by 8-bit standards) and all sorts of weird encounters were just dropped into random regions. Oh, and there were a lot of graveyards. Like, a hell of a lot of graveyards. Honestly, combine all of those graveyards with all of the other bizarre stuff going on in the Minus World of Zelda and it almost seems like a haunted version of the NES classic.
Some folks in the comments have argued that the Minus World appears to be a sort of mashup of the overworld and the dungeon below it, but those tile sets don't really seem to match up. Also, that doesn't really explain the graveyards (So. Many. Graveyards.) that were present in the Minus World but only appeared a couple of times in the original game.
We know that a similar level not intended for play exists within the original Super Mario Bros., and it sounds like SKELUX plans on searching for similar data dumps in other NES games. What would be really cool, though, is if he could speak with some of the original developers and get their take on what, exactly, we're seeing on the screen when these areas have been discovered.