The Midway Geyser Basin's Grand Prismatic Spring is the largest hot spring in the United States, approximately 370 feet in size and around 121 feet deep. But its rainbow waters are what really make it fascinating: While the center of the pool's cerulean hue is pretty characteristic, the deep reds, bright yellows and fiery oranges encircling the edges are not. These colors are caused by pigmented thermophilic bacteria that thrive on the rich minerals produced by the geothermic activity. The color of the bacteria is determined by the temperature of the water – the center is too hot to support life – which is then reflected when light hits the organisms.

This natural wonder is one of the park's most photographed sights, so expect crowds when you visit. You may find fewer visitors if you arrive early, but fog is common until late in the morning on cooler days, so travelers recommend arriving around 10 a.m. No matter when you go, visitors say the attraction's Fairy Falls overlook will provide you with the best views.

Grand Prismatic Spring sits 2 miles southwest of the Lower Geyser Basin and 7 miles north of Old Faithful. Other nearby attractions include the Excelsior Geyser and the Turquoise Pool. Restrooms, a gift shop and multiple parking lots can be found at this attraction, but food and beverage concessions are not available.

The spring is free to visit 24 hours a day between April 20 and Nov. 6 (when roads to the attraction are open). But to access the park, you must buy a seven-day park pass. Additional details can be found on the National Park Service's Old Faithful Area page.