Grand Prismatic Hot Spring, Yellowstone National Park, Wyoming. The striking array of colors reflects different communities of microorganisms that are very sensitive to water temperature. Water flowing away from the center of the spring (blue) cools down, causing a given community of microorganisms to be replaced by a different community that grows best at the new, lower temperature. The boardwalk visible in the lower part of the photo allows tourists to peer into its depths and provides a sense of scale.