The science behind Grand Prismatic Spring’s colors is wilder than the colors themselves

It’s alive, and it’s hot

Grand Prismatic Spring sits in Yellowstone’s Midway Geyser Basin, and it looks like someone spilled a rainbow into the earth.

The thing stretches 370 feet across, wider than a football field, and drops more than 121 feet deep. Every minute, 560 gallons of 160-degree water pour out of it.

It’s the largest hot spring in the United States and the third largest in the world. The colors pull you in first.

What causes them is even better.

Fur trappers saw it first as a “boiling lake”

Back in 1839, fur trappers with the American Fur Company crossed the basin and wrote about a “boiling lake” roughly 300 feet across. That was likely Grand Prismatic.

It took another 32 years before geologist Ferdinand Hayden got a proper look during the 1871 Hayden Geological Survey, the first federally funded exploration of Yellowstone.

He named it “Prismatic” because the colors matched what you see when white light passes through a prism.

The expedition’s reports, William Henry Jackson’s photographs, and Thomas Moran’s paintings helped convince Congress to make Yellowstone the first national park in 1872.

Trillions of bacteria paint those color bands

Those reds, oranges, yellows, and greens ringing the spring aren’t paint or minerals. They come from trillions of thermophilic bacteria, organisms that only survive in hot water.

Different species thrive at different temperatures, so each temperature ring supports its own community.

The bacteria produce carotenoid pigments that work like a natural sunscreen, and those pigments create the color bands you see from above.

Every ring is a different neighborhood of tiny creatures, each one too small to see alone but impossible to miss together.

The deep blue center is almost too hot for life

The center of Grand Prismatic hits about 189 degrees Fahrenheit, close to boiling. At that temperature, almost nothing can survive, so the water is nearly sterile.

That deep blue you see isn’t a pigment. It’s the water itself scattering blue wavelengths of light, the same reason oceans look blue.

As the water spreads outward and cools, it creates rings of different temperatures. Each ring becomes its own mini-ecosystem with a different mix of bacteria, and that’s where the color show begins.

A Yellowstone bacterium helped create COVID-19 testing

In 1966, microbiologist Thomas Brock and his student Hudson Freeze pulled a heat-loving bacterium called Thermus aquaticus out of a Yellowstone hot spring.

The same types of thermophilic bacteria live in and around Grand Prismatic.

An enzyme from that bacterium, Taq polymerase, can copy DNA at high temperatures without falling apart.

Biochemist Kary Mullis used it to develop the polymerase chain reaction, PCR, and won the Nobel Prize in Chemistry in 1993.

PCR now drives DNA testing, crime scene forensics, disease diagnosis, and was central to COVID-19 testing.

The boardwalk puts you right at the edge

A half-mile boardwalk loop crosses the Firehole River and winds through the steaming basin right alongside Grand Prismatic.

You’ll feel the heat rising off the ground and smell sulfur in the air before you even reach the spring. The boardwalk is wheelchair accessible and works for strollers too.

Stay on it. The ground around the spring is dangerously thin, and the water underneath can cause severe burns.

Rangers enforce this strictly, and for good reason.

Climb 105 feet for the full rainbow view

The boardwalk gives you a ground-level look, but the full rainbow pattern only shows itself from above.

The Grand Prismatic Overlook Trail starts at the Fairy Falls Trailhead, about a mile south of the Midway Geyser Basin parking area.

You climb 105 feet over 0.6 miles to a viewing platform that looks straight down on the spring. The round trip runs 1.2 miles and takes about 40 minutes.

The trail sits in a Bear Management Area, so it typically stays closed from March through late May.

Excelsior Geyser next door once blew 300 feet high

Right beside Grand Prismatic sits Excelsior Geyser, one of the largest geyser craters in Yellowstone. In the 1880s, it erupted with blasts reaching 300 feet, among the biggest ever recorded in the park.

Those eruptions hit so hard they wrecked the geyser’s underground plumbing, and it went quiet for nearly a century. Then in September 1985, Excelsior fired back up for about two days, with eruptions reaching 80 feet.

Today it acts as a hot spring, dumping roughly 4,000 gallons of boiling water per minute into the Firehole River.

Turquoise Pool and Opal Pool round out the basin

You don’t have to leave the boardwalk to see the basin’s other thermal pools. Turquoise Pool glows blue-green with water temperatures between 142 and 160 degrees Fahrenheit.

A Hayden expedition named it back in 1878. Opal Pool is smaller and occasionally erupts as a geyser, though those eruptions come rarely and without warning.

Its colors shift with the light, geothermal activity, and seasons.

All four thermal pools, Grand Prismatic, Excelsior, Turquoise, and Opal, line the same single boardwalk loop.

A crashed drone still sits at the bottom

In August 2014, a tourist flew a camera drone over Grand Prismatic Spring and lost control. The drone crashed into the water and sank.

The National Park Service tried to recover it using a helicopter and ground teams, but it’s still down there. The Park Service had banned all drones from national parks just two months earlier.

Scientists raised concerns that the drone could leak pollutants and damage the fragile bacterial communities living in the spring. The incident shows exactly why the rules exist.

Midday sun and warm days bring out the best colors

On cool mornings, steam blankets the surface and hides everything. You’ll see white haze instead of color.

Sunny midday conditions with direct sunlight give you the best view of those bands. The Midway Geyser Basin parking lot is small and fills fast between 10 a.m. and late afternoon, so plan accordingly.

The Fairy Falls Trailhead lot for the overlook is tight too. Arrive before 9 a.m. if you want a spot.

The boardwalk and overlook trail generally stay open from late May through early November.

The colors shift with the seasons, and NASA is watching

Grand Prismatic’s color bands shift slightly through the year as bacterial mats grow and die back.

The spring itself has looked roughly the same for thousands of years, fed by geothermally heated groundwater rising through cracks in the earth.

NASA scientists study Yellowstone’s hot springs because similar environments may have supported the earliest life on Earth. Mars and Jupiter’s moon Europa could hold similar hot spring systems.

Grand Prismatic remains one of the most visited and most photographed natural spots in the country.

Walk the boardwalk at Grand Prismatic Spring in Wyoming

You can reach Grand Prismatic Spring in the Midway Geyser Basin, between Madison Junction and Old Faithful on the Grand Loop Road’s west side. A seven-day vehicle pass to Yellowstone runs $35.

Walk the boardwalk loop for the ground-level experience, then drive a mile south to the Fairy Falls Trailhead for the overlook.

If you have extra time, the Fairy Falls Trail continues 2.5 miles to a 200-foot waterfall, one of the tallest in the park. Old Faithful is just a short drive south.

This article was created with AI assistance and human editing.

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