The Ghost of Slumber Mountain is a 1918 film written and directed by special effects pioneer Willis O'Brien, produced by Herbert M. Dawley, and starring both men. It is the first film to show live actors and stop-motion creatures together on the screen and is often cited as a trial run for The Lost World. An advertising tagline for the film read "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 film attracted the interest of Watterson R. Rothacker, founder of the Industrial Motion Picture Company, who was so impressed by the film's special effects, he joined forces with O'Brien to create the dinosaurs of The Lost World (1925), a classic that advanced the development of special effects techniques.The Ghost of Slumber Mountain originally took up 3000 feet of film and three reels, equivalent to approximately 40 minutes.

However, after the film premiered at the Strand Theater, manager Walter Hayes ordered Dawley to cut the film down to about one reel because it was too long. At one point, the film was cut down to 12 minutes by the producer, and for years this shortened version was the only one thought to exist, but it was eventually restored back to 19 minutes. The rest of the footage is presumed to be lost. The film is available as a supplementary material item on the 2017 The Lost World Blu-ray disc from Flicker Alley. The supplementary material includes The Ghost of Slumber Mountain in a new 2K HD restoration by the "Dinosaur Museum", although one source lists the film's running time at less than 14 minutes. The film as presented on the Flicker Alley Blu-ray runs thirteen minutes after the opening restoration credits are finished.

Researcher Christopher Workman wrote "It has been suggested that the missing footage was deemed too homosexual in subtext, an idea borne out by a scene that did get restored....(in which) Jack tries to persuade Joe to remove his clothes and pose as a faun, but Joe refuses... because there are too many mosquitoes around." (Oddly, the actor who played Joe in the film is unknown.)

Barnum Brown, the renowned dinosaur hunter who discovered Tyrannosaurus rex, was a technical advisor on the film. O'Brien based the dinosaur models used in the film on the paintings of paleoartist Charles R. Knight.Producer Dawley had hired O'Brien to write, direct, co-star and produce the effects for Slumber Mountain, for a budget of $3,000. The collaboration was not a happy one, and Dawley wound up cutting the 45-minute film down to 12 minutes and claimed credit for much of O'Brien's pioneering effects work on the film. Dawley then used a lot of the cut effects footage in a sequel Along the Moonbeam Trail (1920) and his 1923 documentary Evolution. O'Brien received very little financial reimbursement from the project.