The Incredible Petrified World (1957)

When a diving bell goes missing along with its crew of four, their expedition gives up hope of finding them alive. However the missing vehicle has become trapped in a labyrinth of underwater caverns. After exiting their craft, the foursome encounters a survivor from a shipwreck who informs them that there is no escape from their underwater tomb. Our intrepid explorers, however, trace the flow of oxygen to a volcanic vent and a possible escape route.