One of the U.S.’s most-prized natural attractions is Yellowstone National Park’s Grand Prismatic Spring, the largest hot spring in the U.S. and the third-largest in the world. It’s approximately 370 feet in diameter and more than 120 feet deep, but its most distinguishing feature is its striking coloration: Its 160-degree water is a brilliant shade of blue in the center, then it turns from yellow to orange to red around the edges. The rainbow colors are caused by different bacteria and heat-loving algae.