Yellowstone National Park sits on top of the world's largest caldera. Just a few miles underground, sections of still-molten rhyolite magma create thousands of hot spots in the Earth's crust. Fortunately, there have been no recent magma eruptions, but the area has been shocked by frequent earthquakes.
A USGS team from California takes readings and measurements that would indicate change in any underground activities. A rock's composition, its temperature, confining pressure and water content determine whether it exists as a solid of liquid. These underground conditions create Yellowstone's marvelous sights and sounds, making it an active and intriguing tourist destination. Here, the buffalo graze along the Firehole River, not knowing what lies beneath.