The Grand Prismatic Spring in Yellowstone National Park is the largest hot spring in the United States, and the third largest in the world,[1] next to those in New Zealand. It is located in the Midway Geyser Basin. The vivid colors in the spring are the result of pigmented bacteria in the microbial mats that grow around the edges of the mineral-rich water. The bacteria produce colors ranging from green to red; the amount of color in the microbial mats depends on the ratio of chlorophyll to carotenoids. In the summer, the mats tend to be orange and red, whereas in the winter the mats are usually dark green.[5] The center of the pool is sterile due to extreme heat.

The deep blue color of the water in the center of the pool results from a light-absorbing overtone of the hydroxy stretch of water.[6] Though this effect is responsible for making all large bodies of water blue, it is particularly intense in Grand Prismatic Spring because of the high purity and depth of the water in the middle of the spring.