The 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 approximately 70 feet in diameter and over 121 feet deep. The spring discharges an estimated 560 US gallons of 160 °F (70 °C) water per minute.
The vivid colors in Grand Prismatic 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 and on the temperature of the water that favors one bacterium over another. In the summer, the mats tend to be orange and red, whereas in the winter the mats are usually dark green. The center of the pool is sterile due to extreme heat.