Grand Prismatic Spring is the largest hot spring in Yellowstone National Park and the third largest in the world, after Frying Pan Lake in New Zealand and Boiling Lake in Dominica. It lies in the Midway Geyser Basin beside the Firehole River in north-western Wyoming, about 110 m (370 ft) across and, depending on the survey quoted, between 37 m (121 ft) and 50 m (160 ft) deep. It discharges an estimated 2,100 litres (560 US gallons) of water at about 70 °C (160 °F) every minute, and its rings of red, orange, yellow, green and blue come from heat-loving microbes, not from minerals. The best way to understand it is to follow the river that drains it, from its source on the Continental Divide down to Madison Junction, and then to walk the spring itself from its orange rim to its blue centre.
Madison Lake to Old Faithful: The Firehole’s First Miles
The Firehole River rises in Madison Lake on the Continental Divide and flows north for about 34 km (21 miles) before it meets the Gibbon River. It is one of the two main headwaters of the Madison, and so of the Missouri, and early trappers named it for the steam that makes the river look as though it is smoking. In its upper reach it is a cold mountain stream. It drops over Kepler Cascades, south of Old Faithful, and then enters the Upper Geyser Basin, where geothermal water starts to pour in from both banks.
The Upper Geyser Basin holds the densest cluster of thermal features in the park, including Old Faithful, which was named on 18 September 1870 by the Washburn–Langford–Doane Expedition and erupts on average every 92 minutes. The basins along the Firehole sit on flat valley floors between old rhyolite lava flows, which oozed across this ground long after the caldera-forming eruption of about 631,000 years ago. Beneath the basins lie glacial till and gravels left when ice buried the area between roughly 70,000 and 14,000 years ago. Those porous deposits act as storage tanks, letting rain and snowmelt soak down, warm against heated rock and rise again.
The heat itself comes from far deeper. Salty brine 2,400 to 4,800 m (1.5 to 3 miles) below the surface, inside the hot but mostly solid rock that surrounds Yellowstone’s magma chamber, is superheated beyond 204 °C (400 °F) and kept liquid by pressure. Convection in that brine, and conduction through the fractured rock above it, warms the layer of fresh groundwater that eventually surfaces in the Firehole basins. The park as a whole has an estimated 10,000 thermal features, and a 2011 study counted 1,283 geysers that have erupted there, about 465 of them in an average year.
Biscuit Basin and the 2024 Black Diamond Pool Explosion
Downstream of Old Faithful, still within the Upper Geyser Basin, the river passes Black Sand Basin and Biscuit Basin, both counted as part of the Upper Geyser Basin. In 2024 a hydrothermal explosion at Black Diamond Pool in Biscuit Basin damaged the boardwalk and forced the area to close. The pool has kept erupting sporadically, and as of December 2025 Biscuit Basin remained closed to the public. The blast showed that the water feeding Grand Prismatic belongs to a pressurised system. Because Yellowstone’s basins sit so high, water there boils at about 93 °C (199 °F) rather than 100 °C, so a small drop in pressure can flash it to steam.
By this point the river is already carrying a heavy load of thermal water. Temperatures in the Firehole have been measured as high as 30 °C (86 °F), and on average it runs 5 to 10 °C warmer than it does upstream of the geyser basins. Its cold tributaries, among them the Little Firehole River, Iron Spring Creek and Sentinel Creek, give trout a refuge in midsummer.
Crossing the Firehole into Midway Geyser Basin
Between the Upper and Lower basins the river runs past Midway Geyser Basin, a narrow strip much smaller than its neighbours, with 59 geysers. Its outflow, though, is enormous. Visitors cross the Firehole on a footbridge and climb onto a sinter terrace beside the outflow of Excelsior Geyser Crater, which pours 15,100 to 17,000 litres (4,000 to 4,500 gallons) of 93 °C water a minute straight down orange-streaked channels into the river.
Excelsior was once a true geyser. Yellowstone superintendent P. W. Norris, who named it in 1881, described eruptions of 30 to 90 m (100 to 300 ft) that turned the river, nearly 100 yards wide at that point, into "a foaming torrent of steaming hot water". It erupted again in 1888 and in 1890, then went largely quiet, probably because the violence of those blasts damaged its plumbing. Its last notable eruptions came over 46 hours from 14 to 16 September 1985, reaching up to about 24 m (80 ft). The crater, roughly 60 by 90 m (200 by 300 ft), now boils as a hot spring. The same small basin also holds Turquoise Pool and Opal Pool, and, a short walk from Excelsior, Grand Prismatic.
From Orange Runoff to Blue Centre: Walking Grand Prismatic’s Rings
Grand Prismatic does not erupt. Its water rises through the pool and spills outward in a thin sheet over gently sloping terraces of sinter, then drains towards the Firehole. That shallow, steady overflow sets up a temperature gradient, leaving the pool at about 70 °C and cooling as it spreads across the runoff aprons, and each temperature band is colonised by a different community of microbes.
Walk in from the outside and the order is clear. On the outer aprons, where the water has cooled most, mats of thermophilic bacteria and archaea turn rust red and orange. Their colour depends on the ratio of green chlorophyll to orange and red carotenoid pigments, and out here the carotenoids dominate. Closer in, where the water is hotter, the mats grade into yellow, and a green fringe borders the open water; how much of each colour shows depends on the balance of the two pigment types and on how hot the runoff is. The colours also shift with the seasons: in summer the mats tend towards orange and red, while in winter they are usually dark green.
At the centre nothing grows at all. The deep blue there is simply the intrinsic colour of water, seen through tens of metres of clear, sterile liquid. Because microbes cannot survive at the vent, the water stays transparent, and the depth of the pool makes the blue strongest in the middle. The result is a target of colour: blue, green, yellow, orange, red, arranged in rings that follow the temperature gradient from the hottest water outward.
The Fairy Falls Trail View and the 1839 "Boiling Lake"
From the boardwalk, steam rising off the hot water can hide much of the pattern. The best view is from the hillside above the basin, reached by the Fairy Falls Trail, where an overlook looks down on the whole spring and its rings.
The spring has been described for nearly two centuries. In 1839 four trappers from the American Fur Company crossed the Midway Geyser Basin and recorded a "boiling lake" about 90 m (300 ft) across, almost certainly Grand Prismatic. The Washburn–Langford–Doane Expedition passed in 1870 and noted a 15 m (50 ft) geyser nearby, later named Excelsior. It was geologists with Ferdinand Hayden’s 1871 survey, the first federally funded survey of the region, who named the spring for its colours. Their reports helped persuade Congress to create Yellowstone, signed into law by President Ulysses S. Grant on 1 March 1872 as the first national park in the United States. UNESCO added it to the World Heritage List in 1978.
Downstream to the Fountain Paint Pots, Mushroom Spring and Madison Junction
Below Midway the river enters the Lower Geyser Basin, the largest in area at about 28 km² (11 square miles), where features are more spread out. Here are the Fountain Paint Pots, acid mud pots of grey clay, and Great Fountain Geyser, whose eruptions reach 30 to 61 m (100 to 200 ft). Near Great Fountain and White Dome Geyser lies Mushroom Spring, where in 1969 Thomas Brock and Hudson Freeze of Indiana University described a new bacterium, Thermus aquaticus. Its heat-resistant enzyme, Taq polymerase, made the polymerase chain reaction practical, the DNA-copying technique for which Kary Mullis shared the Nobel Prize in Chemistry in 1993. The microbes that colour Grand Prismatic’s rim are the same kind of heat-tolerant life.
Nez Perce Creek, the last of its significant tributaries, joins in this stretch, and the river then cuts through Firehole Canyon, dropping over the Cascades of the Firehole and Firehole Falls. When explorers arrived in the 1830s there were no trout above those falls. Brook trout were stocked in 1889, brown trout in 1890 and rainbow trout in 1923, and since stocking stopped in 1955 the populations have been wild. At Madison Junction the Firehole meets the Gibbon to form the Madison River, which runs about 295 km (183 miles) to Three Forks, Montana, where it joins the Jefferson and Gallatin to form the Missouri.
That route carries Grand Prismatic’s overflow, by way of the Madison and the Missouri, all the way to the Mississippi and the Gulf of Mexico. To see how this water is heated in the first place, read about the Yellowstone supervolcano; for the geyser at the head of the valley, see Old Faithful; and for a hot-spring landscape built by chemistry rather than microbes, compare the white travertine of Pamukkale in Turkey.