Georges Méliès accidentally created 3D film in 1903, nineteen years before the première of the first deliberate one.
This story, like so many, begins with Thomas Edison being a jerk. The early film-maker and Cinemagician Georges Méliès was the most successful and influential auteur of 19th and early 20th century film. I cannot stress how much modern cinema owes to this guy: time lapse, slow motion, the stop trick, multiple exposures, colour film (albeit painted by hand)… he wrote, directed, produced, and starred in his own films, he built his own sets, and built his own cameras too. Arguably, he was the one to introduce the whole idea of story in film – before Méliès, it was mostly shots of workers leaving factories and trains arriving in stations.
If you’re familiar with Méliès, you probably know his famous 1902 film A Trip to the Moon. It was extremely popular in his native France, all around Europe, and also in the United States. But Méliès himself did not profit from its American popularity, because it was pirated by the Edison Manufacturing Company and shown without credit (or cash) to Méliès.
In response to this blatant rip-off, Méliès opened an American branch and sent masters of his films there as soon as he could. Being a tinkerer, he figured out that the fastest way to do this was with a special camera of his own design: it was effectively two cameras connected by a single hand-crank. (Early cameras were hand-cranked, by the way.) One cast, one story, one scene, but two lenses, two cameras, two reels of film… one reel for the European market and one for the American market. That would show Edison!
Méliès began doing this around 1904. Fast forward a hundred years; film researchers at Lobster Films discovered something remarkable. Because those cameras were next to each other, the American and European prints were filmed at ever-so-slightly different angles. And because the cameras were cranked together they were perfectly in sync. Combine the prints together and you have stereoscopic film. You have 3D film.
I’ve never seen this myself, but several Méliès films have been screened in 3D: The Oracle of Delphi, The Infernal Cauldron, and The Mysterious Retort. The effect is apparently remarkable, and the story behind it even more so.