Men already have lived experimentally for weeks at a time on the bottom of the ocean. Both sea floor laboratories and military bases are being planned or, in a few cases, installed. Sea floor mining complexes are in the conceptual design stage. It is only a matter of time before recreational “aquotels” are built safely below the sea’s restless surface. Private sports submarines are an actual, though costly, reality. It is not inconceivable that in the not-too-distant future human beings may overflow the land into complete, self-sufficient communities below the oceans. [Illustration: _In 1965 the U. S. Navy conducted a 45-day experiment in its “Man-In-The-Sea” program in which 10 aquanauts lived and worked 205 feet below the surface of the sea off La Jolla, California. Their undersea base was Sealab II shown at her christening._] [Illustration: _Sealab II shown during final checkout before descent. The aquanauts conducted experimental salvage operations, marine research, and underwent a series of physiological and human performance tests._] NUCLEAR ENERGY’S ROLE The role of nuclear energy in the study, exploration, and utilization of the world ocean is best defined by citing the specific oceanographic interests of the U. S. Atomic Energy Commission (AEC): Development of better instruments and devices for work and study in the ocean, development of ever-stronger national sea power, conversion of seawater to fresh water, possible modification of ocean boundaries, purely scientific studies to advance knowledge, and, indirectly at least, improving the state of oceanographic engineering. Among the technological products of the nuclear age are radionuclides, neutron sources and other radiation sources, radioisotope heat and electric generators, and nuclear reactors. All these are applied to ocean-related endeavors. Several divisions of the AEC have important oceanic interests. These range from pure oceanographic research to development of specific instruments, nuclear reactors, radioisotopic power sources, and other devices for use in or under the ocean. The AEC also conducts extensive marine environmental studies to monitor the effects or ensure the safety of specific projects involving nuclear energy. A statistical summary of specific AEC programs in oceanography is shown in Table I on page 14. Radionuclides in the Sea Before we can follow the atom down into the sea, we must understand something about the potentials, both good and bad, of this incursion of one of our most advanced technologies into one of earth’s least understood environments. This adventurous probing has ramifications for studying both man-produced radioactivity in the sea and the ocean itself as an uncontaminated environment.