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.