During the last Ice Age, millions of large animals roamed the Earth, from wooly mammoths and giant sloths to cave lions and saber-toothed cats. But as the temperatures rose, three-quarters of these species died out. What happened? Can environmental changes alone really explain this mass extinction, or did humans—who at this very time were beginning their conquest of the planet—play a key role? To find out, researchers around the world are hunting and studying fossils in their search for answers to solve the mystery of the Ice Age giants.