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..