Nuclear power plants like this one in Middletown, Pa., currently supply 20 percent of the nation's electricity. Some legislators want them to provide more, but it's turning into quite a sticky subject. Carolyn Kaster/AP hide caption

toggle caption Carolyn Kaster/AP

Coal-fueled power stations like this one in Warrington, United Kingdom, are said to be contributing to global warming. But e-mail messages stolen from a group of climate scientists recently are fodder for skeptics who say global warming is a plot, not a real problem. Christopher Furlong/Getty Images hide caption

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North America was once home to mammoths, mastodons (above) and beavers the size of black bears. Barry Roal Carlsen/University of Wisconsin-Madison hide caption

toggle caption Barry Roal Carlsen/University of Wisconsin-Madison

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