Far north of the Arctic Circle, a nuclear bomb test, dubbed Operation Experiment, is conducted. Prophetically, right after the blast, physicist Thomas Nesbitt muses, "What the cumulative effects of all these atomic explosions and tests will be, only time will tell". Sure enough, the explosion awakens a huge fictional carnivorous dinosaur known as the Rhedosaurus, thawing it out of the ice where it had been hibernating for 100,000 years.
The monster starts making its way down the east coast of North America, sinking a fishing ketch off the Grand Banks, destroying another near Marquette, Canada, wrecking a lighthouse in Maine, and crushing buildings in Massachusetts. The monster eventually comes ashore in Manhattan, and after tearing through power-lines attacks the city. The monster's rampage causes the death of 180 people, injures 1,500 and does $300 million worth of damage.
Arriving on the scene, the military troops of Col. Jack Evans, blast a bazooka hole in the monsters throat and drive it back into the sea. Unfortunately, it bleeds all over the streets, unleashing a "horrible, virulent" prehistoric germ, which begins to contaminate the populace, causing even more fatalities. The germ precludes blowing the monster up or burning it, lest the contagion spread. Thus its decided to shoot a radioactive isotope into the monster's neck wound with hopes of burning the beast up from the inside, killing it.
When the beast comes ashore and attacks the Coney Island amusement park, military sharpshooter Corporal Stone takes the potent radioactive isotope launcher (its the only one of its kind outside of Oak Ridge so he can't miss), and climbs onboard a roller coaster. Riding the coaster to the top of the tracks so he can get to eye-level with the giant beast, he fires the isotope into the monsters wound. The creature lets out a horrible death scream and crashes to the ground dead.
- Paul Hubschmid – as Professor Tom Nesbitt (as Paul Christian)
- Paula Raymond – as Dr. Lee Hunter
- Cecil Kellaway – as Dr. Thurgood Elson
- Kenneth Tobey – as Colonel Jack Evans
- Donald Woods – as Captain Phillip Jackson
- Ross Elliott – as Professor George Ritchie
- Jack Pennick – as Jacob Bowman
- Lee Van Cleef – as Corporal Stone
Beast From 20,000 Fathoms was the first film to feature a giant monster awakened or brought about by an atomic bomb detonation and to attack a major city. Due to its financial success at the box office, it helped spawn the entire genre of "giant monster" films of the 1950s. Producers Jack Dietz and Hal E. Chester got the idea to combine the growing paranoia about nuclear weapons with the concept of a giant monster after the successful theatrical re-release of King Kong in 1952. In turn, this craze inspired the Godzilla series.
When the short story of the same title by Ray Bradbury was published in The Saturday Evening Post, Dietz and Chester were reminded by someone that both works share a similar theme of a prehistoric sea monster, and a lighthouse being destroyed. The producers who wished to share Bradbury's reputation and popularity, bought the right to Bradbury's story and changed the film's title. The movie was promoted as being "suggested" by a Ray Bradbury story. Bradbury would eventually change the title of his story to The Fog Horn when it was reprinted.
Creature effects were assigned to Ray Harryhausen, who had been working with Willis O'Brien, the man who created King Kong, for years. The monster of the film looked nothing like the Brontosaurus-type creature of the short story. A drawing of the creature was published along with the story in the The Saturday Evening Post. At one point there were plans to have the Rhedosaurus snort flames, but this idea was dropped before production began due to budget restrictions. However, the concept was still used in the films movie poster artwork.
Some early preproduction conceptual sketches of the Rhedosaurus showed that at one point it was to have a shelled head and at another point was to be a beaked Dinosaur creature. 
While trying to identify the Rhedosaurus, Professor Tom Nesbitt goes through the dinosaur drawings of Charles R. Knight, a man whom Harryhausen claims as in inspiration. Incidentally, Knight died in 1953, the year Beast was released.
The dinosaur skeleton in the museum sequence is artificial; it was obtained from storage at RKO Pictures where it had been constructed for Bringing up Baby (1938).
This movie had a production budget of $210,000. It grossed roughly $5 million dollars at the Box Office. Original prints of Beast were sepia toned.
The original music score was composed by Michel Michelet, but when Warner Brothers purchased the film they had a new score written by David Buttolph. Ray Harryhausen had been hoping that his film music hero Max Steiner would be able to write the music for the picture, as Steiner had written the landmark score for King Kong, and Steiner was under contract with Warner Brothers at the time. Unfortunately for Ray, Steiner had too many commitments to allow him to do the film, but fortunately for film music fans, Buttolph composed one of his most memorable and powerful scores, setting much of the tone for giant monster music of the 1950s.
- Suite of the film score re-recorded on "Monstrous Movie Music" label (sound samples available)
- Ray Harryhausen: The Beast Master Speaks
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