On 3 May 2003 a tornado smashed through two suburbs of Oklahoma City. It had struck at the height of the tornado season, yet residents were still shocked by the destruction it wrought. Eight thousand homes were destroyed, a billion dollars of damage was wreaked and 40 people lost their lives. What had hit Oklahoma City that day was not just any old tornado - it was a super-twister. Tornadoes are classified on the Fujita scale, or F-scale. Most tornadoes that occur around the world can be classified on the lower reaches of the scale - the F0s, F1s or F2s. These can still cause damage and have winds in excess of 160km/h. Supertwisters are an altogether different beast. Terrifying and destructive, at their most extreme they are powered by wind travelling in excess of 480km/h. This is strong enough to lift strong framed houses from their foundations and seriously damage even reinforced concrete. Every year hundreds of people around the world are killed or injured by supertwisters.