At the bottom of the lagoon, below the point of detonation, Navy divers months later found that the explosion had scooped out thousands of tons of mud and coral sediment, creating a shallow basin half a mile wide. This basin, in the slow settling of returning sludge, became an area from which long-lived radioactivity entered Bikini’s biological system. First Assessments In 3 weeks of final work after Test Baker, the Bikini scientific teams took from the islands and the lagoon many hundreds of samples of plants, corals, crabs, fish, plankton, and water. They noted that radioactivity was present in all samples taken from every part of the atoll, which indicated an early uptake of radionuclides by the biota[9] and suggested that there was a continuing circulation of radioactive debris in the water. They took samples of fish in the open ocean outside the atoll and made comparative collections at other atolls. The instruments and techniques for analyzing radioactivity were far from refined, but all available evidence pointed to the need for more particular efforts to examine radiobiological results. The Bikini Resurvey of 1947 A resurvey of Bikini, the first of many, was conducted with heavy radioenvironmental emphasis in July 1947, a year after the Crossroads tests. The scientific expedition was supported by 2 vessels and included 70 scientists and several hundred Navy personnel. [Illustration: _Bikini Beach as it appeared in the years after Operation Crossroads._] The resurvey group, entering an oceanic environment that had been completely undisturbed for nearly a year, established at once that traces of residual radioactivity still were cycling in Bikini’s ecosystem. For 6 weeks the scientists probed every realm of the atoll environment, sampling biota, making inventories of plant and animal communities, and obtaining core samples from the lagoon floor. When the data had been assembled and reviewed and the reports filed, months later, there was consensus that Bikini had produced no evidence that radioactivity, as a separate and identifiable factor, was having any immediate effect on the health of the atoll, and probably no cumulative effect, either. There were, of course, unknowns. So long as radioactivity remained in the biological cycles there were possibilities of future developments. In 1947 no other place on earth offered an opportunity to observe the natural processes by which radiation contamination is eliminated from an environment. It therefore seemed prudent to compile a longer record, consisting of other, purely radiobiological surveys at Bikini.