"When the supervolcano at Yellowstone last erupted 640,000 years ago, the magma and ash ejected from the caldera covered most of the United States west of the Mississippi river and part of northeastern Mexico. Another such eruption could threaten civilization."<p><a href="http://en.wikipedia.org/wiki/Risks_to_civilization,_humans_and_planet_Earth#Supervolcano" rel="nofollow">http://en.wikipedia.org/wiki/Risks_to_civilization,_humans_a...</a>