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Marine engineer Bob Gage is hired to supervise the construction of an underwater city being built by Dr. Halstead of the Institute of Oceanography. Gage would prefer to devote his energies to the conquest of outer space and is at first skeptical that the project might eventually provide a haven in the event of an atomic war. Nevertheless, he gradually becomes more enthusiastic over the underwater effort, largely because of his growing love for Dr. Halstead's niece, Dr. Monica Powers. When frogmen have completed assembling the prefabricated parts and cells of the city, Gage, Monica, and several others prepare for the arrival of an inspection team from Washington. Gage then learns that the city has been built at the edge of a subterranean chasm that is slowly being washed away, and he orders an immediate evacuation. Although Dr. Halstead and several others lose their lives when the city collapses, Gage discovers that one of the city's cells has survived the disaster. Now convinced that a new underwater city can be satisfactorily built, he makes plans with Monica to begin another project.