Yellowstone National Park contains the most concentrated array of hot springs and geysers in the world. The largest, and one of the most spectacular of these springs in the park is the Grand Prismatic Spring. It is 90 meters (300 feet) across and 50 meters (160 feet) deep. In the centre of the pool the water is 87° C - too hot to support life. In the cooler water along the edges of the pool, however, colonies of thermophilic (heat-loving) cyano-bacteria and algae thrive.

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 colours ranging from green to red; the amount of colour 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 they are usually dark green.

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