It is little known that lying underneath one of America's areas of outstanding natural beauty - Yellowstone Park - is one of the largest supervolcanoes in the world. The impact of a Yellowstone eruption is terrifying to comprehend - huge areas of the USA would be destroyed, the US economy would probably collapse, and thousands might die.

The threat of climate change caused by human activity could turn out to be a minor problem by comparison with a scarcely acknowledged natural hazard. Geologists say there is a real risk that sooner or later a supervolcano will erupt with devastating force, sending temperatures plunging on a hemispheric or even global scale. A report by the BBC Two programme Horizon on one supervolcano, at Yellowstone national park in the US, says it is overdue for an eruption.