The book traces the development of underwater craft from early experimental diving bells and primitive submersibles through pioneering inventors and Civil War experiments, recounting milestone devices and the emergence of torpedoes. It explains technical principles—hull forms, propulsion, periscopes, and safety apparatus—while comparing competing designs and firms with photographs and diagrams. One imaginative chapter offers a speculative voyage inside a modern boat. Final sections examine mines, tactical uses and blockades, questions about neutral shipping, common accidents and rescue methods, and the evolving role of submarines in contemporary naval warfare.