Admiral Nelson (Walter Pidgeon) takes his new nuclear sub for a test run when, surprise, surprise, the Van Allen radiation belt gets set on fire, causing the Earth to heat up in an alarming fashion. He comes up with a plan to solve this minor problem and in true American style it involves blowing stuff up with missiles. In his quest he is backed up by Captain Crane (Robert Sterling) and Dr. Susan Hiller (Joan Fontaine) who is a guest on the sub, investigating what happens to people's mental health under severe pressure.
Seeing as though I only watched this for Joan Fontaine, it's a bit disappointing where she's concerned. Despite being the film's leading lady she doesn't get a close up for the first half hour. Saying that, there's not much to get a close up of. She spends the whole film looking like a local dignitary opening a new museum of which they have zero knowledge or interest. To say that she is distant is an understatement.
The film creates a suitably under water atmosphere due to the sets and the miniatures. It shows the magic of film when some dubious model work and shaking the camera around can create the sense of a crew working under the sea. Saying that there is a shocking scene where an octopus attacks a few members of the crew who are out in their diving gear. They basically roll themselves up in its tentacles and look a bit scared. The editing is partly to blame as it lingers for an age on a useless long shot. When it cuts quickly between close up shots of the action, things improve immeasurably and it actually looks fairly convincing.
If you like this you could also try:
The Day the Earth Caught Fire, The Towering Inferno, The Poseidon Adventure.