Grand Prismatic Springs, Yellowstone National Park, Wyoming, United States

Yellowstone's largest hot spring is the Grand Prismatic Spring. This feature is 370-feet in diameter and more than 121-feet in depth.

Hot springs are the most common hydrothermal features in Yellowstone. Beginning as precipitation, the water of a hot spring seeps through the bedrock underlying Yellowstone and becomes superheated at depth. An open plumbing system allows the hot water to rise back to the surface unimpeded. Convection currents constantly circulate the water, preventing it from getting hot enough to trigger an eruption.

Many of the bright colors found in Yellowstone's hydrothermal basins come from "thermophiles" —microorganisms that thrive in hot temperatures. So many individual microorganisms are grouped together—trillions!—that they appear as masses of color.

Different types of thermophiles live at different temperatures within a hot spring and cannot tolerate much cooler or warmer conditions. Yellowstone's hot water systems often show distinct gradations of living, vibrant colors where the temperature limit of one group of microbes is reached, only to be replaced by a different set of thermophiles.