Which Yellowstone hot spring is famous for its vivid rainbow-colored rings?

The Yellowstone hot spring famous for its vivid rainbow-colored rings is Grand Prismatic Spring.

It lies in the Midway Geyser Basin and is Yellowstone’s largest hot spring. Its name was chosen for the striking colors that resemble the dispersion of white light through an optical prism: blue, green, yellow, orange, and red.

The colors are biological as well as geological. Heat-loving microorganisms called thermophiles form mats around the spring’s cooler edges. Different temperatures favor different communities and pigments, while the deep blue center mainly reflects the natural color of clear water and the pool’s depth.

Grand Prismatic Spring was noted by the Hayden Geological Survey in 1871. It is often confused with nearby Excelsior Geyser, another Midway Basin feature, but Excelsior is a geyser crater rather than the famous rainbow-ringed spring.