Zombies are everywhere, with blockbuster TV shows like The Walking Dead and in Game of Thrones, and films such as 28 Days Later, World War Z, Zombieland, and many others. That popularity stems directly from George R. Romero’s 1968 film Night of the Living Dead. A new video essay from Kristian Williams delves into how one mistake with the film’s release led to the renaissance of zombie stories that terrify and entertain us.

That popularity is due in part to the fact that when Night of the Living Dead was released, its distributor forgot to place a copyright indicator when it changed the title from Night of the Flesh Eaters to its current moniker. According to copyright law at the time, leaving that symbol and the year off meant that it entered the public domain. “The film’s entry into the public domain became the ultimate distribution tool,” says Williams, because theaters, video stores, and TV stations could air it at no cost.

Here, you can watch it right now on YouTube, or download it off of the Internet Archive:

Copyright laws changed a decade after the film hit theaters, so leaving the mark off an original work now wouldn’t allow this to happen. Williams explains out that Romero’s undead were far different from the original zombies from Haitian folklore, and that it was original enough that a copyright would have protected the concept of the modern zombie. Indeed, had the film not entered the public domain, Romero’s hold on the film and concept would have lasted through at least 2024.

Because of the error, Zombies became a thing that could easily be exploited by other creators. Williams notes that if Romero retained the copyright through 2024, we wouldn’t get works like Shaun of the Dead, Michael Jackson’s Thriller, Game of Thrones, The Walking Dead, and the hundreds of other works that have proliferated in the years since.