Supervolcanoes are loosely defined as volcanoes that can eject more than 1,000km3 of material during an eruption - around 1,000 times bigger than a normal volcano. These eruptions measure magnitude 8 or more on the Volcano Explosivity Index.
In any given century, there is around a one in 500 chance one of the world's supervolcanoes will erupt. The consequences of such an eruption would depend on where it took place. Not every supervolcano on Earth has been mapped, but there are thought to be several dozen in existence.