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The working title of this film was The Giant Ymir. The film opens with an offscreen narrator explaining: "Great scientific advances are often times sudden accomplished facts before most of us are even dimly aware of them. Breathtakingly unexpected, for example, was the searing flash that announced the atomic age. Equally unexpected was the next gigantic stride when men moved out of his very orbit to a point more than 20 million miles to earth." Although the ^Var review lists Thomas Henry's character as "Major McIntosh," he is addressed as "General McIntosh" throughout the film. Hollywood Reporter news items add Sid Cassell, Darlene Fields, Terence Maples, John Sorintino, Michael Garth, Jerry Riggio, Saverio LoMedico and Noel Drayton to the cast, but their appearance in the released film has not been confirmed.
According to several August 1956 Hollywood Reporter news items, location filming was done around Rome, Italy in September 1956. According to a modern source, William Hopper was the only principal actor involved in the Rome footage. The modern source adds that the illusion of the creature was created by using a rubber and metal puppet about one foot tall that was moved, one frame at a time, by hand. The special effects scenes were filmed one month after the live action sequences were completed. According to the source, special effects wizard Ray Harryhausen appeared as an extra in some of the zoo scenes.