The Yellowstone Supervolcano

Yellowstone National Park, established by Congress in 1872 as the world's first national park, sits atop one of the largest active volcanic systems on earth. The Yellowstone supervolcano has erupted catastrophically three times in the last 2.1 million years, with the most recent major eruption occurring 640,000 years ago. A full eruption today would deposit ash several feet deep across most of the continental United States and trigger a volcanic winter that would affect global agriculture for years. The park's geysers, hot springs, and fumaroles, including Old Faithful, are surface expressions of this enormous underground heat source.

Source: National Park Service