Some 66m years ago Earth was hit by a space rock r eckoned to have been 10km across. The resulting chaos caused the extinction of dinosaurs and many othe r species, opening the way for the age of mammals－ and ultimately humans. It also left a big hole in what is now southern Mexico. That hole is one of only three known of similar dimensions (the other two are Vredefort in South Africa and Sudbury in Canada). How many other craters of a dimension similar to the one in southern Mexico are known to people?