Grand Prismatic Spring
Grand Prismatic Spring is the star attraction of the Midway Geyser Basin and is very appropriately named: it is indeed grand, and as you can see above, its waters and bacterial mats feature a veritable rainbow of colors. With dimensions of 250 feet by 380 feet, it is the largest hot spring in Yellowstone, and third largest in the world (the two larger ones are in New Zealand.) I was absolutely mesmerized by Grand Prismatic, and upset that I couldn't get closer to it to check it out better!
As with other hot springs, the colors represent variations on the temperature of the water, and the impact this has on microbial life. In the center of the spring the water is very hot; nothing lives there, and the color blue dominates. This fades to a slightly cooler greenish ring, which is then followed by a yellow band. Runoff channels go in all directions, and are colored orange, red and brown by thermophilic algae and bacteria.
The huge size of the spring actually makes it rather difficult to photograph, unless you are lucky (or rich) enough to be able to shoot it from a helicopter. From above, the spring looks like a giant blue eye rimmed green and yellow, and surrounded by thick orange "eyelashes".