Ancient Cauldron
By Alice J. Dunn, National Geographic Magazine

Most people who are familiar with Yellowstone National Park have heard of Old Faithful, Mammoth Hot Springs, and Mud Volcano. These are just some of the more than 10,000 thermal features in the park. But did you know that Yellowstone National Park sits within the Yellowstone caldera, one of the world's largest active volcanoes?

The extreme size of the caldera (approximately 28 by 47 miles [45 by 76 kilometers) is why most people are not aware of it. The caldera erupted in a series of massive explosions (some 2 million, 1.3 million, and 630,000 years ago) that dwarf any volcanic eruptions in recent history. In fact, the largest of the three eruptions (2 million years ago) was at least 2,500 times larger than the eruption of Mount St. Helens in 1980.

The hot spot that was the force behind those eruptions is what powers the thermal features that make Yellowstone so well known today. And as evidence that all of this activity is still ongoing and ever changing, a portion of Yellowstone's Norris Geyser Basin was temporarily closed this summer due to increased thermal activity and high ground temperatures.