By the mid 1940s, Mallinckrodt Chemical Works had run out of space to store the radioactive waste left behind, so in 1946 they began to ship the leftovers to a relatively underpopulated area north of St Louis, next to a creek by the name of Coldwater. It was here that approximately 250,000 barrels of radioactive material were dumped in shallow pits and exposed to the elements.
According to a 1990 article in the New York Times, the toxic waste was dumped secretly with the approval of the federal government.
<irony>Conspiracy theory! To believe this you would have to believe that this has remained a secret for 69 years, involving dozens of people across government and industry! We all know that cannot happen!</irony>