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Grand Prismatic Spring is Yellowstone's largest hot spring, roughly 200 to 330 feet across according to the National Park Service and more than 121 feet deep, and is often ranked third largest in the world. Its rings of orange, yellow, and green come from heat-tolerant microbial mats that thrive at different temperatures moving out from the scalding blue center. The colors are most vivid on sunny days, and many visitors hike the short Fairy Falls trail to a viewpoint above the spring for the full rainbow effect that's hard to see from boardwalk level.
Visitor guide
Grand Prismatic Spring, in Yellowstone's Midway Geyser Basin, is the park's largest hot spring and its most photographed. Its deep blue centre is ringed by bands of green, yellow, orange and red, a colour pattern that comes from living microbial mats rather than from minerals in the water.
Sources differ on the exact dimensions. Wikipedia and an older NPS page give a diameter of about 370 feet (roughly 110 metres), while a current NPS place page gives a range of 200 to 330 feet and a depth of more than 121 feet; Wikipedia cites about 160 feet. Whichever figure you use, it is the largest hot spring in Yellowstone, and Wikipedia ranks it third largest in the world, behind Frying Pan Lake in New Zealand and Boiling Lake in Dominica. It discharges roughly 560 US gallons (about 2,100 litres) of water per minute, which Wikipedia puts at around 160 degrees Fahrenheit (70 degrees Celsius).
The NPS credits fur trapper Osborne Russell with an 1839 description of this spring, which it calls the earliest described thermal feature in Yellowstone that can be definitely identified. The name comes from the 1871 Hayden Geological Survey, whose geologists chose it for the spring's spectrum of colours. The NPS notes that the basin was once known as Hell's Half Acre and that it holds two of Yellowstone's largest hydrothermal features, Grand Prismatic Spring and Excelsior Geyser.
The bands are made by thermophiles, heat-loving microbes that form mats along the edges of the spring. The NPS explains that colourless and yellow microbes grow in the hottest water while orange, brown and green ones grow in cooler water, so the colours sort themselves by temperature as the runoff cools. The balance between chlorophyll and carotenoid pigments shifts with the seasons, and the mats tend to look orange and red in summer and dark green in winter. The blue centre is the natural colour of the water itself, most visible where the spring is deepest and too hot for mats to form.
From the ground, the Midway Geyser Basin boardwalk brings you close to the steam and the coloured margin, but the full pattern is hard to read from that angle. For a view from above, the NPS describes the Grand Prismatic Overlook Trail as an easy 1.2-mile round trip from the Fairy Falls trailhead, climbing about 105 feet to a viewpoint over the spring and the basin, and notes that parking there is very limited. The trailhead is about a mile south of Midway Geyser Basin.
Stay on the boardwalks and designated trails; the NPS prohibits off-trail travel in this area, including on the hills overlooking the spring. Check the NPS for current access, trail conditions and parking. Because parking is tight, arriving by private vehicle with a driver, as on a Voyzo private tour, means your group is dropped at the trailhead and picked up without circling for a space.
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