The surface waters of the seas, down to depths of 200 meters, are areas of rapid mixing in which temperature, density, and salinity are almost uniform. Below the surface water is a zone in which temperature decreases and density and salinity increase with depth. This zone, known as the thermocline, may reach a depth of 1000 meters. Because density is increasing here, vertical motion is reduced, and exchanges between the surface and the deep waters are impeded. Knowledge of temperature, density, and salinity is important to understanding what happens to radionuclides in the ocean. Physical conditions affect the rates of physical movement of radioactivity in the mixed (surface) layer, the degree to which radionuclides are held at the thermocline, and the processes by which radionuclides pass the thermocline and enter the deep-water cycles and upwellings. [Illustration: _Men aboard the research vessel_ Shimada _pulling in plankton nets during sampling operations at sea_.] The surface currents of the ocean are largely wind driven and their patterns generally well known. New concepts of the vertical and horizontal diffusion of substances introduced into the ocean were developed, however, in studies of ocean-borne fallout during and after nuclear tests in the Pacific. The first of these surveys was conducted near Eniwetok and Bikini. Scientists aboard a Navy vessel sampled water and plankton to depths of 300 meters at some 90 points spread over an area of 78,000 square miles to determine the disposition of early fallout from the nuclear detonations. Some weeks later another expedition voyaged from Eniwetok to Guam and returned, covering an area of 375,000 square miles to follow (by sampling) the mass of water-borne radioactivity resulting from the test and to note the intervening effects of diffusion, dilution, biological uptake, and decay. In 1958 two more surveys were conducted, the first to ascertain the spread and depth—with samplings below the thermocline—of a radioactively tagged water mass immediately following an underwater detonation, and the second to follow the westward drift of the tagged water mass. Significantly, it was found that plankton immediately take up large amounts of radioactivity. Planktonic forms, in fact, proved to be the most sensitive indicators of the presence of radioactivity in the marine environment. Further, the daily vertical migrations of plankton—down in response to sunlight and up at night—seemed a part of the process by which radionuclides move from the upper waters to the deeps. The expedition scientists noted that the masses of low-level radioactivity moved in the ocean significantly slower than the surface currents, a circumstance attributable in large measure to biological factors. The distribution of residual radioactivity in the sea a month after the close of a nuclear testing program could be determined by counting radioactivity in plankton samples.