This map from the U.S. Geological Service shows the range of the volcanic ash that was deposited after the biggest of the Yellowstone National Park eruptions around 2.1 million years ago. "These eruptions left behind huge volcanic depressions called â€œcalderasâ€ and spread volcanic ash over large parts of North America," it said. "If another large caldera-forming eruption were to occur at Yellowstone, its effects would be worldwide. Thick ash deposits would bury vast areas of the United States, and injection of huge volumes of volcanic gases into the atmosphere could drastically affect global climate. Fortunately, the Yellowstone volcanic system shows no signs that it is headed toward such an eruption in the near future. In fact, the probability of any such event occurring at Yellowstone within the next few thousand years is exceedingly low."