Within Yellowstone National Park lie over half of the whole world's geothermal features. Perhaps not too surprising in that much of the park lies within the caldera of a supervolcano. They provide an astonishing array of visual delights. The volcano last erupted 640,000 years ago and is due to do so again, certainly sometime in the next 100,000 years, probably destroying all that you see here - a humbling thought. The northern hemisphere will be in trouble too from the fallout !