The Grand Prismatic Spring in Yellowstone National Park is the largest hot spring in the United States, and the third largest in the world, next to those in New Zealand.
There is no doubt that many Native American peoples were aware of the Grand Prismatic Spring long before Europeans first saw it, but the first records of the Spring are from early explorers and surveyors.
In 1839, a group of fur trappers from the American Fur Company crossed the Midland Geyser Basin and made note of a "boiling lake", most likely the Grand Prismatic Spring, with a diameter of 300 feet (91 meters).
In 1870, the Washburn Party, led by General Henry D. Washburn, visited the Spring, noting a 50 foot (15 meter) geyser nearby (later named Excelsior.
The vivid colors in the spring are the result of pigmented bacteria in the microbial mats that grow around the edges of the water. The bacteria produce colors ranging through greens, yellows, oranges, and reds; 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. The center of the pool is sterile because of its 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. Though this effect is responsible for making all large bodies of water blue, it is particularly intense here as a result of the high purity and depth of the water in the middle of the spring.
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