The largest hot spring in the United States, and the third largest in the world — at 370 feet across, it is wider than a football field. The view from the boardwalk in Midway Geyser Basin is striking but limited; to see the full rainbow of colours you need elevation. The Grand Prismatic Overlook Trail, about a mile south of the main parking area, reaches a ridge with a view over the entire spring. The hike is short (about a mile return) but fills up fast in summer — arrive early or come on a weekday.
Grand Prismatic Spring
A story
The vivid rings of colour around the Grand Prismatic Spring are not mineral deposits but living organisms — colonies of heat-loving bacteria called thermophiles, each species occupying a narrow temperature band as the water cools from the 190°F centre to the edges. The brilliant oranges, yellows, and reds at the margins are caused by carotenoid pigments, similar to those that colour autumn leaves.