THE BEAST FROM 20,000 FATHOMS

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". The explosion awakens a 200-foot (61 m) long carnivorous dinosaur known as a Rhedosaurus, thawing it out of the ice where it had been held in suspended animation for millions of years. Nesbitt is the only surviving witness to the beast's awakening and later is dismissed out-of-hand as being delirious at the time of his sighting. Despite the skepticism, he persists, knowing what he saw.

The dinosaur begins 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 destroying buildings in Massachusetts. Nesbitt eventually gains allies in paleontologist Thurgood Elson and his young assistant Lee Hunter after one of the surviving fishermen identifies from a collection of drawings the very same dinosaur that Nesbitt saw. Plotting the sightings of the beast's appearances on a map for skeptical military officers, Elson proposes the dinosaur 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 after his bell is swallowed by the beast, which eventually comes ashore in Manhattan. It devours a police officer shooting at it, totals cars, knocks over buildings, and generally causes a panicking frenzy. A later newspaper report of its rampage lists "180 known dead, 1500 injured, damage estimates $300 million".

Meanwhile, military troops led by Colonel Jack Evans attempt to stop the Rhedosaurus with an electrified barricade, then blast a hole with a bazooka in the beast's throat, which drives it back into the sea. Unfortunately, it bleeds all over the streets of New York, unleashing a horrible, virulent prehistoric contagion, which begins to infect the populace, causing even more fatalities. The infection precludes blowing up the Rhedosaurus or even setting it ablaze, lest the contagion spread further. It is decided to shoot a radioactive isotope into the beast's neck wound with hopes of burning it from the inside, while at the same time neutralizing the contagion.

When the Rhedosaurus comes ashore and reaches the Coney Island amusement park, military sharpshooter Corporal Stone takes a rifle grenade loaded with a potent radioactive isotope and along with Nesbitt climbs on board the Coney Island Cyclone roller coaster. Riding the coaster to the top of the tracks to get to eye-level with the beast, he fires the isotope into its open neck wound. It thrashes about in reaction, causing the roller coaster to spark when falling to the ground, setting the amusement park ablaze. With the fire spreading rapidly, Nesbitt and Stone climb down as the park becomes engulfed in flames. The Rhedosaurus collapses and eventually dies from the isotope's radiation poisoning.

The Rhedosaurus is killed after a radioactive isotope is fired into its wound at Coney Island, stopping both the beast and the contagion it spread.

The climax exemplifies Cold War-era fears, as scientific ingenuity counters both the monster and the unforeseen biological threat resulting from its rampage. The conclusion ties the dangers of atomic experimentation to the unpredictable consequences it can unleash, suggesting humanity's control over science may be far more fragile—and costly—than it appears.