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Grand Prismatic Spring in Yellowstone National Park is the largest hot spring in the United States, and the third largest in the world. It is located in the Midway Geyser Basin. Grand Prismatic Spring was noted by geologists working in the Hayden Geological Survey of 1871, and named by them for its striking coloration. Its colors match most of those seen in the rainbow dispersion of white light by an optical prism: red, orange, yellow, green, and blue. The bright, vivid colors in the spring are the result of microbial mats around the edges of the mineral-rich water. The mats produce colors ranging from green to red; the amount of color in the microbial mats depends on the ratio of chlorophyll to carotenoids and on the temperature gradient in the runoff. In the summer, the mats tend to be orange and red, whereas in the winter the mats are usually dark green. The deep blue color of the water in the center of the pool results from the intrinsic blue color of water. The effect is strongest in the center of the spring, because of its sterility and depth. This aerial photo shows Grand Prismatic Spring from the south in August 2022.Photograph credit: Carsten Steger