An underwater team circles the globe to test an earthquake warning system.

Dr. Doug Standish, his assistant Dr. Craig Mosby, Dr. Phil Volker, Dr. Maggie Hanford, Hank Stahl, and Dr. Orin Hillyard make up a team of specialists in undersea life assembled for an expedition aboard the Hydronaut , an atomic-powered submarine. Their mission is to place electronic sensors at strategic places on ocean beds throughout the world to provide an early-warning system for the volcanic eruptions which have caused a worldwide series of earthquake disasters. Volker has consented to join the mission on the condition that a sidetrip be made to locate a sunken treasure containing a cache of valuable crystals. The presence of Hanford, the only female crew member, is an additional source of tension on board the Hydronaut . Then Hillyard is injured when he tries to set a sensor near an active volcano, and Stahl narrowly escapes injury from a giant eel. Their job finally completed, the scientists head for the sunken treasure. Just as they find it, they receive an urgent order by radio to place one last sensor near a newly-discovered volcano; and they are forced to abandon the treasure. The volcano erupts as they are fixing the sensor, and the submarine is trapped on the ocean bed by falling rock. In desperation, Standish blows off half of the ship with dynamite, with the result that the other half rises safely to the surface. A helicopter comes to their rescue, but they are unable to return for the treasure.