As the scorpion attacks and derails the passenger train, the label "Lionel Lines" is clearly seen on the side of the locomotive's coal tender.
A typical Willis O'Brien touch - in a long shot of the cage descending into the cave, a tiny stop motion bat flies across the screen. Only O'Brien would add another day's work to a scene where it would be barely noticed.
That giant worm with the "octopus-like arms" seen in this film is a prop from the unused spider pit sequence from the original King Kong (1933).
The volcano shown at the beginning was Paricutin which erupted in 1943 and was active for about a decade.
Willis H. O'Brien and Pete Peterson began filming the special effects of this film in a large remodeled dressing room at the Tepeac Studios in Mexico City, but when money became tight they finished the picture in Peterson's garage in Encino, California.