Springfield Coal is a series of coal seams under southern Illinois, Kentucky and Indiana. Miners have known them for years and now, after all that digging, paleontologists are studying what's been left behind. They have to go down… and look up, because the largest fossilized forest ever discovered is on the roof of a massive coal mine. One scientist calls it "the closest thing you'll find to time travel." Bill DiMichele, curator of fossil plants at the Smithsonian Museum of Natural History, is one of those who discovered the 300-million-year-old forest.