The smallest of Yellowstone’s geyser basins, Midway Geyser Basin (also dubbed “Hell’s Half Acre”) actually contains two of the park’s largest hydrothermal features: Grand Prismatic Spring and Excelsior Geyser, which dumps 4,000 gallons of water a minute into neighboring Firehole River. The spring’s psychedelic coloration comes from pigmented bacteria in the surrounding microbial mats. The amount of color in the mats depends on the water temperature and the ratio of chlorophyll (green pigment) to carotenoids (yellow to red pigment).
In the summer, the mats burn orange and red, while winter turns them a dark green. The spring’s lurid blue “eye” remains sterile because of its extremely high heat.