Far north of the Arctic Circle, a nuclear bomb test, dubbed Operation Experiment, is conducted. Prophetically, right after the blast, physicist Thomas Nesbitt (Paul Christian) muses, "What the cumulative effects of all these atomic explosions and tests will be, only time will tell." Sure enough, the explosion awakens a 10-metre (33 ft) tall, 30-metre (98 ft) long carnivorous diapsid known as the Rhedosaurus, thawing it out of the ice where it had been hibernating for 100 million years. The only witness to the beast's awakening, Tom Nesbitt, is dismissed as delirious, but he persists.
The Beast 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. Nesbitt gains allies in paleontologist Thurgood Elson (Cecil Kellaway) and his lovely young assistant Lee Hunter (Paula Raymond) after one of the surviving fishermen identifies from a collection of drawings the same dinosaur as Nesbitt saw. Plotting the sightings of the Beast on a map for skeptical military officers, Elson proposes the Beast is returning to the Hudson River area where fossils of Rhedosaurus were first found. In a diving bell search of the undersea Hudson River Canyon, Professor Elson is killed by the Beast. The Beast eventually comes ashore in Manhattan. A newspaper report of the Beast's rampage lists "180 known dead, 1500 injured, damage estimates $300 million".
Arriving on the scene, military troops led by Col. Jack Evans (Kenneth Tobey) stop the Beast with an electrified barricade, blast a bazooka hole in the Beast's 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 Beast up or burning it, lest the contagion spread. Thus it is decided to shoot a radioactive isotope into the Beast's neck wound with hopes of burning the Rhedosaurus up from the inside, killing it.
When the Beast comes ashore and attacks the Coney Island amusement park, military sharpshooter Corporal Stone (Lee Van Cleef) takes a rifle grenade loaded with a potent radioactive isotope, (the only one of its kind outside of Oak Ridge, so pressure is on him not to miss), and climbs on board a rollercoaster. Riding the coaster to the top of the tracks so he can get to eye-level with the Rhedosaurus, he fires the isotope into the Beast's wound. The Beast lets out a horrible scream, thrashes about setting the park ablaze and finally crashes to the ground in its death throes.
Paul Christian as Professor Tom Nesbitt
Paula Raymond as Lee Hunter
Cecil Kellaway as Dr. Thurgood Elson
Kenneth Tobey as Colonel Jack Evans
Donald Woods as Captain Phil Jackson
Ross Elliott as George Ritchie
Steve Brodie as Sgt. Loomis
Jack Pennick as Jacob Bowman
Michael Fox as ER doctor
Lee Van Cleef as Corporal Jason Stone
Frank Ferguson as Dr. Morton
King Donovan as Dr. Ingersoll
James Best as Charlie, radar operator
The film had a production budget of $210,000. It earned $2.25 million at the North American box office during its first year of release and ended up grossing over $5 million. Original prints of Beast were sepia toned.
The short story "The Beast from 20,000 Fathoms" by Ray Bradbury was published in The Saturday Evening Post in 1951. When Dietz and Chester were negotiating with Bradbury to rewrite their screenplay, he reminded them that both works shared a similar theme of a prehistoric sea monster and a lighthouse being destroyed. The producers, who wished to share Bradbury's reputation and popularity, promptly bought the rights to his story and changed the film's title. The film credits list "Screen Play by Lou Morheim and Fred Freiberger, Suggested by the Saturday Evening Post Story by Ray Bradbury."
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 Harryhausen, 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.
In a scene attempting to identify the Rhedosaurus, Professor Tom Nesbitt rifles through dinosaur drawings of Charles R. Knight, a man whom Harryhausen claims as an inspiration. 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).
The climactic roller coaster live action scenes were filmed on location at The Pike in Long Beach, California and featured The Cyclone Racer entrance ramp, ticket booth, loading platform, and views of the structure from the beach. Split-matte in-camera special effects by Harryhausen effectively combined the live action of the actors and coaster background footage from The Pike parking lot with the stop-motion of the Beast destroying a model of the coaster.
The Beast from 20,000 Fathoms was the first live-action film to feature a giant monster awakened or brought about by an atomic bomb detonation to attack a major city. Due to its financial success, it helped spawn the 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 a successful theatrical re-release of King Kong. In turn, this craze included Them! the following year about giant ants, the Godzilla series from Japan that has spawned movies from 1954 into the present day, Behemoth, the Sea Monster (UK 1959, US release entitled The Giant Behemoth) and Gorgo (UK 1961).
In the 2008 monster movie Cloverfield, which also involves a monster terrorizing New York City, inserts a frame from The Beast from 20,000 Fathoms(along with frames from King Kong and Them!) into the hand held camera footage used throughout the film.
The film was nominated for AFI's Top 10 Science Fiction Films list.