Seismologists have discovered a massive magma reservoir beneath the Yellowstone supervolcano in Wyoming, US, that suggests its volcanic system could be more than 5.6 times larger than was previously thought. Although it was already known that Yellowstone had one magma reservoir, located about 5-16km (3-10 miles) below the surface, the new study, published in Science, has revealed another, much larger reservoir sitting directly below the first, located around 20-50km (12-30 miles) below the surface. This reservoir is thought to have a volume of around 46,000 cubic km ?" compared to a volume of around 10,000 cubic km for the shallow reservoir. To make their discovery scientists analysed the vibrations made by earthquakes that passed beneath the volcano. The technique not only sheds light on this volcano's potentially life-threatening eruptions but it could also help us understand other volcanoes such as Calbuco, which is currently erupting in Chile.