Yellowstone is one of the world's largest, most active and most explosive volcanoes. It hasn't looked much like a volcano since the last huge explosion, 650,000 years ago, which blew out a huge area, covering a lot of the States with ash.
Nowadays, the volcano is a massive magma chamber, lying just below the Earth's crust. There are clues to its existence - boiling pools of water, coloured like rainbows by sulphur and heat-loving algae; bubbling mud ponds; cracks in rock which puff out eggy smelly gases, killing plants and trees. Over 200 geysers exist - some erupt predictably, others may wait years then explode. The land itself "breathes", rising and falling in tune with the molten rock's pressure and flow.
When the super volcano next erupts, some say that will be the end of us all. But until then, it is a spectacular place to visit, and to watch bison warming themselves in the steam from the pools as if It were Bison Spa Day!
