The scale of life beneath our feet

Scientists have estimated the total amount of life on Earth that exists below ground. It is made up mostly of microbes, such as bacteria and their evolutionary cousins, the archaea; and it represents a lot of carbon - about 15 to 23 billion tonnes of it. That is hundreds of times more carbon than is woven into all the humans on the planet. Bob Hazen, executive director of the Deep Carbon Observatory, spoke with our science correspondent Jonathan Amos.