Local differences in dose rates at three feet The highest levels measured were generally under trees, particularly under Pandanus trees where litter had accumulated. The highest levels at Rongelap islet in 1955 were measured in the remaining palm-frond huts where fallout remained trapped in the roof and the wall thatching. Readings one inch above the floor were lower than those at three feet above the ground and readings close to the walls and roof were highest of all. There were relatively high readings over some open areas where soil algae were abundant. The soil algae form a crust roughly one centimeter thick and retain most of the radionuclides from fallout. Return to background level The return to background level, < first, as would be expected, 0.02 mr/hr, occurred in the intertidal zone, except for a few small areas of beach rock covered with a film of algae. Levels of 2 0.02 mr/hr were measured in July, 1957 in the inter- tidal zones at Kabelle and Rongelap islets and in the newly constructed village on Rongelap islet. Construction of the village entailed the removal of the thatched huts and bulldozing of a considerable part of the area.