Mammoth Hot Springs is a large complex of hot springs on a hill of travertine in Yellowstone National Park, Wyoming, US. It was created over thousands of years as hot water from the spring cooled and deposited calcium carbonate. Although these springs lie outside the caldera boundary, their energy has been attributed to the same magmatic system that fuels other Yellowstone geothermal areas. As the first national park in the world, Yellowstone National Park is well-known for its wildlife and many geothermal features. [Photo by Xu Lin / China.org.cn]