This is Yellowstone's largest hot spring, 370 feet in diameter. It's in the Midway Geyser Basin, and you can reach it by following the boardwalk. The spring is deep blue in color, with yellow and orange rings formed by bacteria that give it the effect of a prism. For a stunning perspective, look down on in from the Fairy Falls Trail.
Midway Geyser Basin, off Grand Loop Rd., Yellowstone National Park, Wyoming, United States