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Cracks in the perfect image of DDT appeared when traces of the insecticide began to show up in a wide variety of organisms throughout the world. As more and more people investigated this problem, it became increasingly evident that terrestrial and aquatic animals were accumulating comparatively high concentrations of DDT from extremely low levels in their environment. It also became apparent that DDT and all of the other chlorinated hydrocarbon insecticides were not species-specific, but were toxic to all forms of animal life including man. In 1965, when the Great Lakes Fishery Laboratory of the U.S. Bureau of Commercial Fisheries began to monitor pesticide residues in fish from the Great Lakes, it was discovered that the fish contained not only DDT, but also dieldrin, another chlorinated hydrocarbon insecticide. Fish from Lake Michigan in particular contained relatively high levels of both of these insecticides; concentrations of DDT were in the parts per million (ppm) range, a factor at least several million times greater than the few parts per trillion found in the water. Two questions presented themselves: first, How did these insecticides get into the water? and second, How did the fish build up such high concentrations in their bodies from such low concentrations in the water?