Millions of people visit Yellowstone each year to see its geysers, fumaroles, hot springs, and mud pots. It's the largest concentration of thermal features in the world. The park sits on top of the world’s largest active volcano. Called the Super Volcano. Its most recent eruption was more than 600,000 years ago. All that remains is the top, or caldera.
When you come into the Park they’ll give you a map and it has an overlay of the caldera. It’s huge.