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. Yellowstone National Park was the world’s first national park, set aside in 1872 to preserve the vast number of geysers, hot springs, and other thermal areas, as well as to protect the incredible wildlife and rugged beauty of the area. Yellowstone lies on top of a gigantic hotspot where light, hot, molten mantle rock rises towards the surface.

Subsequently, the park contains half of all the world’s known geothermal features, with more than 10,000 examples of geysers and hot springs. Over the past 17 million years or so, this hotspot has generated a succession of violent eruptions including a dozen or so super eruptions. The last full-scale eruption of the Yellowstone volcano happened nearly 640,000 years ago.