Did you know that exactly 140 years ago Yellowstone National Park was established? This first national park in the world is well known for its geothermal features like hot springs (http://goo.gl/8xBxM), geysers (http://goo.gl/BYs3J), fumaroles (http://goo.gl/ndiZ9) etc.
Those geothermal remains are just blurry footprints of powerful supervolcano eruptions. Today Yellowstone Caldera (aka Yellowstone supervolcano) is still considered an active volcano. Could you image how another eruption would look like? (hint, caldera is about 34 by 45 miles, 55 by 72km).