An engineer, a psychologist and several other disparate types take part in an experiment to see if people can live for extended periods of time in a city built under the ocean.
| Actor | Role |
|---|---|
 William LundiganBob Gage | Bob Gage |
 Julie AdamsDr. Monica Powers | Dr. Monica Powers |
 Roy RobertsTim Graham | Tim Graham |
 Carl Benton ReidDr. Junius Halstead | Dr. Junius Halstead |
| CChet DouglasChuck Marlow | Chuck Marlow |
 Paul DubovGeorge Burnett | George Burnett |
 Karen NorrisPhyllis Gatewood | Phyllis Gatewood |
 Kathie BrowneDotty Steele | Dotty Steele |
| EEdward MalloryLt. Wally Steele | Lt. Wally Steele |
 Edmund CobbMeade (uncredited) | Meade (uncredited) |
| GGeorge DeNormandDr. Carl Wendt (uncredited) | Dr. Carl Wendt (uncredited) |
| PPaul PowerAuthor in Visiting Party (uncredited) | Author in Visiting Party (uncredited) |