Back in 1918, special effects artists had to make dinosaurs the old-fashioned way. Creating sculptures out of clay and bringing them to life through stop-motion animation was the only way to go, and the pioneering artist in this medium was Willis O’Brien. He’s probably best known for his work on King Kong—the giant ape and Skull Island’s dinosaurs were his creations—but O’Brien also made a number of silent short films which featured prehistoric creatures, including The Ghost of Slumber Mountain.

The plot is pretty simple. Uncle Jack tells his nephews about the time he went up to Slumber Mountain, (surprise!) fell asleep and dreamed of seeing prehistoric creatures through a magical telescope carried by a fellow named Mad Dick (who was played by O’Brien himself). The whole thing was mainly an excuse to get dinosaurs and other ancient critters on screen—the film’s tagline was: “These giant monsters of the past are seen to breathe, to live again, to move and battle as they did at the dawn of life!”

The short is just a shadow of what it was meant to be. Originally planned to be a feature film, the final product stretched only 18 minutes. Still, the movie was a financial success. According to the folks at Turner Classic Movies, it took about $3,000 to make the film but it brought in over $100,000 in profit. Not too shabby.

Although it’s usually only remembered by film buffs and dinosaur fans today, The Ghost of Slumber Mountain represents a significant milestone in the history of movies because it was the first time live actors were paired with stop-motion dinosaurs. This is the movie that got the ball rolling and gave O’Brien some of the skills he would later use on movies like The Lost World and Mighty Joe Young. Without it, we might never have seen Professor Challenger face dinosaurs on an South American plateau or seen a giant gorilla hang from the Empire State Building.