Professor Iain Stewart continues his epic exploration of how the planet has shaped human history.
Iain explores man's relationship with fire. He begins by embarking on an extraordinary encounter with this terrifying force of nature - a walk right through the heart of a raging fire.
Fire has long been our main source of energy, and Iain shows how this meant that the planet played a crucial role in Britain's industrial revolution, whilst holding China's development back.
Along the way he dives in a mysterious lake in Oregon, climbs a glacier of salt, crawls through an extraordinary cave in Iran and takes a therapeutic bath in crude oil.