Four adventurers descend to the bottom of the sea. When their surface tether snaps, they find themselves marooned in a network of underwater caverns.
| Actor | Role |
|---|---|
 John CarradineProf. Millard Wyman | Prof. Millard Wyman |
 Robert ClarkeCraig Randall | Craig Randall |
 Phyllis CoatesDale Marshall | Dale Marshall |
| AAllen WindsorPaul Whitmore | Paul Whitmore |
 Sheila NoonanLauri Talbott | Lauri Talbott |
 George SkaffDr. J.R. Matheny | Dr. J.R. Matheny |
| MMaurice BernardOld Man in the Caverns | Old Man in the Caverns |
| JJoe MaierhouserJim Wyman | Jim Wyman |
 Lloyd NelsonWilson, Sonar Man | Wilson, Sonar Man |
| HHarry RavenThe Captain | The Captain |
| MMilt CollionHank | Hank |
| LLowell HopkinsReporter | Reporter |
| JJack HaffnerJimmy, Reporter | Jimmy, Reporter |
| RRobert CarrollNarrator | Narrator |