Peter Evans investigates one of the largest volcanic systems on the planet, Yellowstone National Park, which has been dubbed a "super volcano". Only a handful exist in the world, but when one erupts (and an eruption is already overdue) it will be unlike any volcano we have everwitnessed. But it is one of the least understood natural phenomena. A new volcanic observatory in the park hopes to change that. EMAIL: radioscience@bbc.co.uk Producer Alexandra Feachem