132 "The global cataclysm that destroyed the dinosaurs" "In the past 600 million years there have been five major mass extinctions that on average extinguished half of all species. The largest mass extinction to have affected life on Earth was in the Permian-Triassic, which ended the Permian period 250 million years ago and killed off 90% of all species. The last such mass extinction led to the demise of the dinosaurs and has been found to have coincided with a large asteroid impact this is the Cretaceous–Tertiary extinction event. There is no solid evidence of impacts leading to the four other major mass extinctions, though a recent report from Ohio State scientists stated that they have located a km diameter impact crater beneath the East Antarctic Ice Sheet which may date back about 250 million years, based on gravity measurements, which might associate it with the Permian-Triassic extinction event. In 1980, physicist Luis Alvarez, his son, geologist Walter Alvarez, and nuclear chemists Frank Asaro and Helen V. Michael from the University of California, Berkeley discovered
