Now this is truly incredible! A group of stormchasers captured some beautiful and terrifying footage of a supercell thunderstorm developing over Wyoming this weekend. As far as thunderstorms go, supercells are the least common, but they're responsible for most of the violent tornadoes in the U.S. In addition to extreme winds, they also dump torrential rain and hailstones that are bigger than golf balls -- causing flash floods and a whole lot of damage. Their rising, spinning vortices of air -- rotating updrafts called mesocyclones -- can reach speeds of over 100 miles an hour (about 160 km/h) and sometimes last hours.