The Grand Prismatic Spring in Yellowstone National Park is the largest hot spring in the United States, and the third largest in the world,[3] after Frying Pan Lake in New Zealand and Boiling Lake in Dominica.

Grand Prismatic Spring was noted by geologists working in the Hayden Geological Survey of 1871, and named by them for its striking coloration.

Its colors match most of those seen in the rainbow dispersion of white light by an optical prism: red, orange, yellow, green, and blue.

In 1839, a group of four trappers from the American Fur Company crossed the Midway Geyser Basin and made note of a "boiling lake", most likely the Grand Prismatic Spring,[5] with a diameter of 300 feet (90 m).

[6][7] The bright, vivid colors in the spring are the result of microbial mats of thermophilic bacteria around the edges of the mineral-rich water.