Yellowstone is an overdure supervulcano
- Three super-eruptions at Yellowstone appear to have occurred on a 600,000-700,000 year cycle starting 2.1 million years ago. The most recent took place 640,000 years ago - suggesting Yellowstone is overdue for an eruption.
Super-eruptions are so rare - the last one happened in Toba, Indonesia, about 74,000 years ago. Geologists think these eruptions take place about every 50,000 years, which suggests one is overdue.
About 40 supervolcanoes are dotted across the globe. There are two in Britain - one in Glencoe, Scotland, the other in Scafell in the Lake District. However, most supervolcanoes, including those in Britain, burned out long ago.
Yellowstone, located in the western state, Wyoming, is a dormant supervolcano, which means a major eruption could happen in the future.