Volcanoes are usually seen as destructive, but Dr Iain Stewart shows that no force has played a more important role in creating the planet we know today.
A volcano in Ethiopia offers a dramatic illustration of the heat that lies just beneath the Earth's surface - heat that fuels volcanoes, but also drives some of the most fundamental process on the planet.
And in Iceland we discover how the Earth's inner heat drives the system of plate tectonics.
Iain discovers that volcanoes also played a critical role in saving the planet from one of the greatest disasters it has ever faced - Snowball Earth. 700 million years ago the planet completely froze over. A planet completely covered in ice reflects most of the sun's heat back into space, so once the snowball took over the planet it seemed there was no escape. But over millions of years volcanoes kept erupting from under the ice, and gradually carbon dioxide built up in the atmosphere until a super greenhouse effect finally melted the snowball.
Out of this great disaster life on Earth took a giant leap forward. Until this time all life on Earth was simple, single celled bacteria. But within a few million years of the snowball ending, the first complex life had evolved.