The film's stop motion animation special effects were created by Willis O'Brien.
An earthquake strikes Mexico, resulting in the overnight birth of a new volcano. Geologists Dr. Hank Scott and Dr. Arturo Ramos are dispatched to study this crisis. En route to the village of San Lorenzo, the two men witness a destroyed house and a totaled police car. They find a dead policeman nearby, as well as an abandoned and seemingly orphaned infant.
They take the infant to San Lorenzo and give it to friends of the child's now missing parents; they are welcomed by the village's priest, Father Delgado. In addition to the disappearances of locals and the destruction of their homes, there have been wholesale slaughter of livestock and strange roaring noises in the night. The villagers believe that the culprit to be a demon bull and have been pestering Delgado for divine assistance. Undaunted, Hank and Arturo begin their geological survey as members of the Mexican army, led by Major Cosio, arrive in San Lorenzo to begin disaster relief efforts. Hank meets and falls in love with local rancher Teresa Alvarez, and makes friends with a young boy named Juanito.
The volcano erupts again, with the true culprits behind the disappearances and deaths are revealed as giant prehistoric scorpions.
- Richard Denning as Dr. Hank Scott
- Mara Corday as Teresa Alvarez
- Carlos Rivas as Dr. Arturo Ramos
- Mario Navarro as Juanito
- Carlos Múzquiz as Dr. Velasco
- Pascual García Peña as Dr. Delacruz
- Pedro Galván as Father Delgado
- Arturo Martínez as Major Cosio
- Fanny Schiller as Florentina
Willis O'Brien, creator of the stop-motion effects for the original King Kong, was the special effects supervisor, albeit on a smaller budget. Pete Peterson, who worked with O'Brien on Mighty Joe Young and would again on The Giant Behemoth, did most of the actual hands-on animation. O'Brien borrowed heavily from other previous movies for the special effects in this film. The models used for the trapdoor spider and the giant tentacled worm have been reported to be the same ones that were used in the famous "Lost Spider Pit Sequence" from the original King Kong. However, biographers dispute whether O'Brien saved his models, and Ray Harryhausen's An Animated Life noted that many models used in Kong were still in storage at RKO in the 1950s, by which time many were decayed. The sounds made by the scorpions were recycled from the ant sound effects from the movie Them! A large-scale scorpion "head" was used for close-up reaction shots, but the head's human-like features distracted from the realism of O'Brien's animated models.
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.
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 sounds of the scorpions are the same sounds as the ant chirps in Them! (1954).
The volcano shown at the beginning was Paricutin which erupted in 1943 and was active for about a decade.
The trapdoor spider that attacks Juanito in the scorpions' underground home is one of the original models left over from the famous deleted spider sequence in King Kong (1933).
While filming the stop-motion effects at Tepeac Studios, O'Brien and Peterson were assisted with miniature set construction by Ralph Hammeras, who was at the same studio filming the visual effects for The Giant Claw (1957).