The permafrost of Alaska and Siberia tells a story that has puzzled explorers for over two centuries. When you dig into it, half of what you find is the remains of extinct megafauna: bones, twisted ligaments, even pieces of frozen muscle. It looks as though an entire ecosystem was thrown into a gigantic blender and then dumped into the river valleys. For years, mammoth tusks were exported by the thousands from the Yukon, Alaska, and Siberia. Many old pianos from the 1800s still have keys made from that mammoth ivory. These animals did not end up there because of human hunters. They were destroyed in some type of catastrophe, and the most likely trigger, he argues, came from out there.