"The extraordinary Joplin twister the single deadliest tornado since officials began keeping records in 1950 was a rare destructive phenomenon known as a multi-vortex, hiding two or more cyclones within the wider wind funnel.
"Sunday's storm smashed the southwest Missouri city's hospital, left nothing but splintered trees where neighborhoods once stood, and killed at least 116, with the death toll expected to rise. The storm injured another 500 and and damaged or destroyed at least 2,000 buildings.
"Added to the record 875 tornadoes that tore across the country in April, this latest disaster has experts asking why 2011 has spawned so many deadly storms. While researchers suss out the causes for this year's record-breaking season, one thing is certain: Unusually big twisters are blasting through heavily populated areas."