Yellowstone National Park, established on March 1, 1872, is the world's oldest National Park. It is famous for having more than 10,000-plus hot springs and geysers, the majority of the planet's total. (In two days we saw only about 20.)Yellowstone's geothermal features are created by one of the world's largest active volcanoes; its last eruption caused a crater or caldera that spans about half of the parks size.

The area in the photos is Grand Prismatic Spring, Yellowstone's largest hot spring, 370 feet in diameter. It's in the Midway Geyser Basin reached by following a boardwalk. The spring is deep blue in color, with yellow and orange rings formed by bacteria that give it the effect of a prism--more beautiful than photos or words can describe.