There was some documentary's about the super volcanoes a few years back - Yellowstone's erupted 3 or 4 times apparently (If you look at Yellowstone in a satellite image you can follow the line of calderas from the eruptions , basically the whole valley is the caldera
).From what the documentary makers and the scientists are saying its not a case of if Yellowstone goes boom , its just when ,and if its not Yellowstone then apparently there's 1/2 a dozen super volcanoes to choose from.Anyone of them going boom would be enough to throw us into a nuclear winter scenario.