Grand Prismatic Pool, located in Midway Geyser Basin, is the largest hot spring in North America, 75 m by 100 m, and the deepest in Yellowstone, measuring 34 m deep. The Pool can be viewed from the boardwalk accessible from the highway or by following the trail to Fairy Falls, where a hill along the way provides a vantage point at height.

The fault-fractured rock of the region allows rain, snowmelt, and groundwater to percolate downward, become heated, and rise upward to feed the area's hydrothermal features. At Grand Prismatic Pool the water flows outward from the spring evenly across the low mound formed of broad, low, step-like terraces. The deep blue of the hottest water is reflected in the steam rising above the spring. The orange, yellow, and brown colors in the cooler water of the runoff channels are produced by thermophilic cyanobacteria with different species living at different temperatures.