Snowmelt and rainwater, superheated hundreds of feet below the surface, blast from Sawmill Geyser in a geothermal belch as high as 20 feet. Hardened remains of old lava flows cover much of the area.
On August 29, 1870, a 30-year-old Army lieutenant named Gustavus Doane, part of an exploratory expedition in the Yellowstone region in the territory of Wyoming, scrambled his way to the summit of Mount Washburn above the Yellowstone River. Looking to the south, he noticed that something was missing from a stretch of the Rocky Mountains: mountains. For miles and miles, the only elevations were in the distance, forming parentheses around a huge forested basin. Doane saw only one way to explain the void. "The great basin," he wrote, "has been formerly one vast crater of a now extinct volcano."
The lieutenant was right: Yellowstone is a volcano, and not just any volcano. The oldest, most famous national park in the United States sits squarely atop one of the biggest volcanoes on Earth. Doane was wrong, however, in one crucial respect. Yellowstone's volcano is not extinct. To an unsettling degree, it is very much alive.