This is a book about the dramatic periods in the Earth's history called mass extinctions - short periods (by geological standards) when life nearly died out on Earth. The most famous is the mass extinction that happened about 65 million years ago, and that caused the death of the dinosaurs. But that was not the worst mass extinction - that honour goes to the extinction at the end of the Permian Period, about 250 million years ago, when over 90% of life is thought to have become extinct. What caused these catastrophes? In this book, Tony Hallam, geologist and writer, looks at all the different theories and also what the study of mass extinctions might tell us about the future.