Animals migrate, fish migrate, birds migrate, but stones stay as still as… well, as stone. Except they don’t, as poet Alyson Hallett discovered when she climbed "Cader Idris". While she was staring at a rock that looked out of place, a geologist appeared, telling her how that rock had migrated in a glacier and settled far from its motherbed. So she was inspired to make seven journeys around the world with inscribed stones. We join her on her way to Australia with stone number three.

Kate Humble leads a team of scientists into the Danakil Desert in Ethiopia, the hottest place on Earth, to find out how life can survive there. From the geology of the volcanoes that formed the Rift Valley to the primitive organisms that thrive in the baking soil, extreme conditions can teach us a lot about the origins of life. But the local Afar people also have their own lessons for the team.