Professor Iain Stewart explores man's relationship with fire as his examination of how geology, geography and climate have influenced human history comes to a close. For most of Earth's history there was no fire. It was only 400 million years ago that vegetation evolved, allowing fire to spread across the planet. Then a million years ago, early humans became the first, and so far only, animal to control this complex and deadly force. To discover the landmarks in man's increasing mastery of fire, Iain goes diving in a mysterious lake in Oregon, climbs a glacier of salt, crawls through an extraordinary cave in Iran and takes a bath in a puddle of crude oil. The end of this story is yet to be written. Man has now reached a turning point in history where the use of carbon is threatening civilisation. To continue to advance, the link between human progress and the burning of carbon-based fuels has to be broken.