Orange Mound Terrace at Mammoth Hot Springs: Yellowstone National Park, Wyoming

The Orange Mounds of Yellowstone National Park in Wyoming are travertine domes formed by the unhurried flow of a spring through the vents at the top that slowly builds on to the existing mound, shown in the photo here, by depositing dissolved limestone on which the mound sits. Although travertine is usually white, the Orange Mound appears so vividly colored because of bacteria that live on it. In fact, this photo captures just one of its many colorations because it changes colors depending on conditions and bacteria populations.