Of all Yellowstone's thermal wonders, none is more photographed than Grand Prismatic Spring, the largest hot spring in the United States and one of the most colorful natural features on Earth. Seen from above, it resembles a great steaming eye, ringed in bands of vivid color.

Those colors are alive. The deep blue center reflects the near-boiling, sterile water at the spring's heart, while the surrounding rings of orange, yellow, and green come from mats of heat-loving microbes, each thriving at a different temperature as the water cools toward the edges. The result is a living palette painted by microscopic organisms.

The scale is hard to grasp from the boardwalk that skirts its edge, where visitors see mostly steam and a hint of the color beneath. The full effect reveals itself from an overlook trail on the hillside above, from which the entire spring and its blazing rings spread out below.

Like all of Yellowstone's thermal features, Grand Prismatic is as dangerous as it is beautiful. The water is scalding and the crust fragile, and the boardwalks exist to keep visitors safe. Admired from the proper distance, it stands as one of the park's supreme spectacles, a reminder that some of nature's finest art is drawn in heat and microbes.