The Grand Prismatic Spring is situated in the park’s Midway Geyser Basin. It’s the United State’s largest hot spring. It is roughly 300 feet in diameter and about 121 feet deep.
The water here is a veritable collage of colors. Thus, it is said to be the park’s most photographed attraction. The center is cerulean blue and the edges are encircled with fiery oranges and deep reds.
The multi-colored effect is the result of pigmented thermophilic bacteria that live off of the minerals that are generated by geothermal activity. The bacteria’s color is a result of the water temperature. The center of this body of water is a bit too hot to support the bacteria’s life.
Expect crowds when the weather is good. While arriving early is usually a good idea, that is not the case here. There is often a significant amount of fog here in the morning on cooler days. The best time to visit is a little before 10 a.m. Park your vehicle at the Fairy Falls sign then hike to the overlook point. The hike may take you half an hour but there is no better place from where to see it.
You can reach a number of the various thermal features of the worthwhile Lower Geyser Basin via the one-way 3-mile long Firehole Lake Drive. The Firehole Lake Drive can be found off of the Grand Loop between Madison and Old Faithful. You can see hot springs and geysers from both the drive and the boardwalk too.