Yellowstone is a supervolcano- a volcano so enormous and powerful that when it erupts, instead of forming a pointy peak, it blasts away a giant crater in the earth.
Yellowstone's last giant erruption was about 630,000 years ago.
It left a huge oval crater around 50km across and 80km long.
Today, Yellowstone is filled with forests and wildlife.
It also has hot springs, squirting geysers,and fumaroles, where hot gas escapes from the ground.
The area around the caldera is a national park, where tourists visit.
But just under the surface is a vast chamberof molten magmaand gas that could one day cause another eruption.
