Yellowstone National Park is a geologically active area, with its Old Faithful geyser and hot springs such as the Grand Prismatic Spring, seen here. But Yellowstone has also been home to explosive volcanism, and the regularity of its eruptions is troubling to some.

Their concern is based on three eruptions: a gigantic caldera-forming blast that occurred in the region 2.1 million years ago, another that followed 700,000 years later, and a third 700,000 years after that (which might mean that this old faithful is ready to blow right around now). Just in case Yellowstone awakens again, the U.S. Geological Survey's Yellowstone Volcano Observatory keeps a close eye on the park's volcanic and earthquake activity.