Grand Prismatic Spring in Yellowstone National ParkYellowstone is home to thousands of thermal features, but the largest hot spring in the park is the Grand Prismatic Spring. Located in the Midway Geyser Basin, the spring is 370 feet in diameter and 121 feet deep. In addition to being the largest thermal feature in the park, the Grand Prismatic Spring was also the first thermal to be definitively identified (by a white explorer). An 1839 diary entry by a fur trapper named Osborne Russell describes a "boiling lake" of deep indigo blue about 300 feet in diameter. But as anyone can see, the deep blue of the spring's mineral-rich water isn't its only distinguishing color. The vivid rings of rusty reds and vibrant yellows and greens that encircle it are caused by colonies of pigmented thermophiles -- microbes that live within specific extreme temperature ranges at hot springs. The bands of color correspond to the microbes' grades of tolerance. Where one kind of colony reaches its temperature threshold, another type immediately takes over.