My research spans many areas of the earth sciences, especially the inter-relationships between the Earth's changing environments and the evolution of life. A major long-term project involves the causes of mass extinctions, including the end-Cretaceous extinction 65 million years ago when the dinosaurs and many other forms of life died out. The evidence for a large asteroid or comet impact at that time led to work on a general theory of large-body impacts and mass extinctions, research into impact cratering and its environmental effects and into the possible astronomical causes for periodic comet showers in Earth's history. This work has taken me to impact craters and geologic boundaries on six continents. Recently, I have been focusing on the causes of the Permian/Triassic mass extinction (250 million years ago) - the most severe mass extinction of life - with field studies in Europe, Japan and South Africa.