Manson crater

The Manson impact crater is near the site of Manson, Iowa where an asteroid or comet nucleus struck the Earth during the Cretaceous Period, 74 Ma (million years ago). It was one of the largest known impact events to have happened in North America.^[2] Previously it was thought to have led to the extinction of the dinosaurs until isotopic ages proved that it was too old.^[3]

• http://en.wikipedia.org/wiki/Manson crater