A sign is posted in front of a Walmart store in American Canyon, Calif. The state has filed a lawsuit against Walmart for allegedly disposing of hazardous waste, state officials announced. Justin Sullivan/Getty Images hide caption
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Nearly two-thirds of cell mutations that cause cancer are caused by random error, a study found. Steve Gschmeissner/Science Source hide caption
A boy from a tribe in Northern Kenya stands on a dried-up river bed. Christopher Furlong/Getty Images hide caption