Earth's Van Allen radiation belts have caught on fire, causing the global temperature to rise day by day, which threatens to end all life in three weeks. Admiral Harriman Nelson (Pidgeon), who has constructed an experimental nuclear submarine called Seaview, sets out on a mission to restore the planet to its normal state.
The film became the basis for a television series later in the decade.
Voyage to the Bottom of the Tropes:
- Artistic License Astronomy: The Van Allen radiation belt literally catching fire. Enough said (let alone the whole "can be surgically fixed with a nuke" part).
- Berserk Button: Crane explodes when he finds Alvarez giving a sermon to the crew in the Seaview's mess room about how all their efforts will ultimately prove worthless and all that is left for them to do is to make their peace with God. As he puts it, what the crew needs least of all right now is to have some doom-sayer on board bringing their spirits down.
- Climate Change: When the skies start to burn, the ice caps melt while Seaview is in the Arctic Ocean, leaving chunks of ice to rain down on it (which ignores the fact that ice floats).
- Communications Officer: Sparks. He is the first to get very distressed when increments in atmospheric radiation makes it impossible to contact anybody.
- Cool Ship: The Seaview. A fully-armed state-of-the-art atomic submarine that can outrun and outdive regular submarines, has an internal pool for sharks (or other sea creatures worth investigating)... and, oh, yeah, has a glass nose. The fact that it has the In-Universe nickname of "Nelson's Folly" showcases how little people think of its technological advancements.
- Crazy Enough to Work: The plan to nuke the Van Allen belt. Most of the drama between Nelson and almost everybody else comes from the fact that Nelson is the only one that doesn't think this.
- Deus ex Nukina: To stop the world from turning into a dried husk, a nuclear missile must be fired to the Van Allen radiation belts from the Marianas.
- The Fundamentalist: Alvarez believes that it's God's will that Earth should burn up, and is one of the two saboteurs onboard Seaview.
- Giant Squid: A gigantic octopus attacks the Seaview as it is nearing its goal, and is driven away by electricity. Earlier in the film, an actual giant squid attacks the divers repairing the transatlantic phone line on the ocean bottom.
- A Glass in the Hand: One of the many signs that the crew is growing increasingly uncomfortable with the whole situation is when one of them crushes a glass as he hears Admiral Nelson saying that the attempt to communicate with land via the underwater landline in the Atlantic didn't work and, thus, will continue the expedition to the launch point and nobody will be able to talk to their families.
- Hot Sub-on-Sub Action: One of the many rapid-fire catastrophes that get in the heroes' way on the climax is the encounter of a couple of nuclear subs that have been sent to hunt down Seaview.
- Ignored Expert: Nelson, who proposes pulling the Deus ex Nukina (and insists that there is no time to argue), but is undermined by UN Emergency Scientific Meeting head delegate (and scientist) Emilio Zucco, who believes the "sky fire" will burn itself out. Nelson, thus, has to perform the expedition with everybody else thinking he's insane.
- Insufferable Genius:
- Admiral Nelson. It is this, and various decisions that (in Crane's eyes) make him look like The Neidermeyer, that make the Captain a Commander Contrarian. The Seaview is even mentioned early in the film to have been nick-named "Nelson's Folly" by the news.
- Emilio Zucco as well. The United Nations Emergency Scientific Meeting devolves into an intellectual measuring contest with his insistence that his theory is right and Admiral Nelson is bonkers vs. Admiral Nelson's insisting that there's no time to sit down and wait to see if it's true.
- Number Two: Captain Crane, with some degree of Commander Contrarian to Admiral Nelson. For most of the film, he actually hopes that Zucco's theory is the correct one and Nelson is wrong.
- Sea Mine: The submarine almost plows headfirst into a mass of underwater mines, and a tense sequence starts when they have to send in a minisub to remove one they got tangled with that gets blown up when it accidentaly triggers a couple of them. The need to go around it (and thus lose time) adds to Nelson and Crane's It's All My Fault moments.
- Set the World on Fire: Earth's Van Allen belts are on fire and if nothing is done about them, they will slowly roast the planet.
- Stern Chase: The moment Nelson storms out of the U.N. Emergency Scientific Meeting to get the Seaview to the Marianas, the world's governments try to stop him from doing something that they (erroneously) think will destroy the world. The mine field is one of such attempts, and the climax involves the (foreshadowed) attack of various subs ordered to sink Seaview.
- Threatening Shark: Seaview has a pool inside it, where Lucius keeps a shark for his studies. Late in the film, Dr. Hiller, one of the two saboteurs onboard, falls into it after a brief struggle with Capt. Crane and gets eaten.
- Title Theme Tune: Sung by Frankie Avalon, who also plays Lt. Romano.