Grand Prismatic Spring, Excelsior Group, northern Midway Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA in August 2011.

Grand Prismatic Spring is, by far, the most beautiful thing I’ve ever seen. This rainbow-colored hot spring is best appreciated from the boardwalk along the northeastern edge of the pool and from atop Midway Bluff, a hill on the eastern side of the highway next to the Firehole River and from “7622 Hill” on the southeastern side of Fairy Falls Trail/Fountain Freight Road. [7622 Hill is now off limits due to the death of a Taiwanese tourist on the hillside in 2014; a boardwalk leading up to a viewing platform is supposedly being planned.]

The deep blue color at the center of the pool is the result of very hot water having abundant, suspended, <0.5 micron-sized, colloidal silica particles. The yellow, orangish, reddish, and brownish areas are mats of extremophile bacteria (e.g., Phormidium, Synechococcus, Calothrix). The greenish areas are the effect of “blue water” mixing with the light reflecting from the yellowish bacterial mats.