Long before Marvel and DC popularized cinematic shared universes, horror filmmakers and TV producers were pulling disparate stories and characters into crossovers, both in earnest and in jest. While Freddy vs Jason and Alien vs Predator are obvious examples, shared universes have also been implied between A Nightmare on Elm Street and The Evil Dead, The Evil Dead and Friday the 13th, Child's Play and Hellraiser, and The Conjuring and Saw (to name a few). Then, there's the arena of fan theories, a place where even the most outlandish hypotheses seem to carry weight, like: Josh was the real villain of The Blair Witch Project, Home Alone is a Jigsaw origin story, Pennywise is "The Other Mother" in Coraline (among many others).

On fan theory that's been around for a while and refuses to die (you'll catch that pun in a second) is called "The Breaking Dead Theory", and it connects 2 of TV's biggest megahits of the 2000s. The crux of the postulation is that Walter While (played by Bryan Cranston), the complex antihero of Breaking Bad, accidently triggered the zombie apocalypse portrayed in The Walking Dead. If this is the first time you're hearing this theory, I agree it sounds absurd-at first. The more you look at the connections, however, the more plausible the idea becomes. For starters, both shows were flagships of AMC; the network owns both properties, meaning that despite being created by unique teams they can, in theory, establish a canonical connection. Many people believe they have.

I mentioned before this isn't a new theory; it's actually been bandied about since the very beginning of The Walking Dead. In Season 1 Episode 2 ("Guts"), Daryl (Norman Reedus) digs through a bag of dope looking for some pain killers. Eagle-eyed fans immediately spotted a sizable chunk of Walter White's signature "Blue Sky" meth from Breaking Bad. For the uninitiated, meth can be clear or opaque, but it's always a shade of white; blue meth doesn't exist in real life and was created specifically for Vince Gilligan's series. While the connection is undeniable, many continue to argue over whether this was merely an Easter Egg (one show giving a shout-out to another show) or actually implies that White was more than just a morally-challenged family man fighting cancer, but in fact the harbinger of world destruction.

"The Breaking Dead Theory" picked up so much steam a few years ago, Netflix even produced a 4-minute video laying out the case point-by-point. Give it a spin below, and you'll discover blue meth is just the tip of the iceberg!

"What if Breaking Bad was about more than Walter White starting as a bumbling chemistry teacher and turning himself into a full-fledged monster? What if he turned everyone into full-fledged monsters? What if Walter White's "Blue Sky" meth started the zombie epidemic of another all-time great show: The Walking Dead!"

Part of what allows "The Breaking Dead Theory" to persist and proliferate is that fact that Walking Dead creator Robert Kirkman has never revealed the source of the zombie outbreak in his comics, and the AMC series never addresses the issue. Kirkman claims there is, of course, a cause, but it's insignificant to the story. This a has allowed for postulations ranging from government ineptitude to alien invaders-and "The Breaking Dead Theory" actually holds more water than most of them. Then there's the fact that the shows producers seem to enjoy yanking our collective chains.

During a Walking Dead Q&A at San Diego Comic-Con last week, the question of what started the zombie apocalypse came up yet again (as it inevitably does). Producer Gale Anne Hurd told an inquisitive fan "[It was] the meth from Breaking Bad, for sure" to which Kirkman replied, "That's canon, it's confirmed." And if you believe that, maybe you're still holding out hope that Carl didn't really die in Season 8! The Walking Dead returns to AMC for its 9th Season this October.