The 175-square-mile Valles caldera forms a large pock in the middle of northern New Mexico, west of Santa Fe. It last exploded 1.2 million and 1.6 million years ago, piling up 150 cubic miles of rock and blasting ash as far away as Iowa. That's equivalent to roughly 2,000 Mount St. Helens eruptions.

Around the fractured ring of Valles caldera, lava flows from after the major eruptions built up mountains and left obsidian flows as recently as 60,000 years ago. As with other calderas, there are still signs of heat below: hot springs are still active around Valles.

Geologists suspect the cause of Valles caldera has something to do with how the western United States' portion of the North American tectonic plate is being pulled apart. Will Valles erupt again? No one knows.