One of nature's deadliest phenomena, supervolcanos are unlike conventional volcanos in that they are huge, hard-to-detect craters in the ground. When they erupt, it is believed they have the potential to destroy continents. This update of a film originally shown in the Horizon series reveals that the next volcano to erupt - now overdue - lies under America's Yellowstone national park. Producer Mark Hedgecoe (Revised repeat) The new series of Horizon continues on Thursday at 9pm