The Yellowstone volcano is considered to be the largest volcano in the world. It last erupted 70,000 years ago. Most people picture a mountain when they think of a volcano, Yellowstone is almost flat, completely underground. The caldera is over 50 miles wide, and has been rising several inches a year since 2004. An eruption would be very, very bad. It would make Mt. St. Helens look like a child’s pop gun going off.