The Grand Prismatic Spring, with approximately 370 feet (110 m) in diameter and is 160 feet (50 m) deep, which discharges an estimated 560 US gallons (2,100 L) of 160 °F (70 °C) water per minute. It is the largest hot spring in the United States, and the third largest in the world. The 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 center of the pool is sterile due to extreme heat.