Yellowstone National Park

Listen for a moment. Under the hiss of steam and the creak of the boardwalk, there is a low gurgle — water boiling somewhere beneath your feet. The air smells faintly of sulphur, a little like struck matches. Pale vapour drifts across the path, then clears to show ground streaked in orange, white and rust. This is Yellowstone. The earth here is not quite finished, and you can hear it working.

Yellowstone was established in 1872, when President Ulysses S. Grant signed the law that set it aside. It is widely considered the first national park in the world. The idea was new and a little radical. This land would not be sold, logged or claimed by settlers. It would belong to the public, for the enjoyment of everyone. Most of the park lies in the northwest corner of Wyoming, with thin edges reaching into Montana and Idaho.

The scale is hard to hold in your head. The park covers more than two million acres — larger than some small countries. Forests of lodgepole pine roll across high plateaus. Rivers cut deep canyons. Wide valleys open where herds of bison graze. You could spend weeks here and still miss whole regions. Most visitors follow a long loop road that links the main areas, and even that road takes a full day to drive without stopping.

Now, the reason the ground steams. Yellowstone sits on top of one of the largest volcanic systems on Earth. Hundreds of thousands of years ago, a massive eruption emptied the magma chamber below. The ground then collapsed into an enormous basin, called a caldera. You may be standing inside it right now and not realise it, because the caldera is so large that its rim is hard to see from the ground. Molten and partly molten rock still sits beneath the surface, and that heat drives everything you see.

Here is the first surprise. Yellowstone holds more than ten thousand hydrothermal features — hot springs, mud pots, steam vents and geysers. Geysers are rare on our planet. They need a precise mix of heat, water and underground plumbing. Yet this one park contains roughly half of all the geysers in the world. Some erupt every few minutes. Some go quiet for years. Each one has its own rhythm.

The most famous is Old Faithful. It was named in 1870 by members of an early expedition, who noticed how regularly it erupted. It is not the tallest geyser in the park, and it is not perfectly punctual. But it is reliable enough that rangers can predict its eruptions with good accuracy. People gather on the benches, check the posted time, and wait. Then the cone starts to splash... and a white column rises into the sky.

A short distance away lies Grand Prismatic Spring, the largest hot spring in the United States. From above, it looks like a giant eye. The centre is a deep, clear blue. Around it run rings of green, yellow and bright orange. Those colours do not come from minerals alone. They come from living things — mats of heat-loving microbes. Each colour marks a band where the water has cooled enough for a different community to survive.

That leads to the second surprise. In the 1960s, a scientist named Thomas Brock studied the microbes in Yellowstone's hot springs. He found a bacterium that thrives in near-boiling water. Years later, an enzyme from this tiny organism became essential to a laboratory technique for copying DNA. That technique is now used in medical testing, forensic science and genetic research around the world. So a small creature from these steaming pools played a part in modern medicine.

People knew this land long before it became a park. More than twenty-five Native American tribes have historic ties to Yellowstone. They hunted here, travelled through, gathered plants and held ceremonies. One place mattered especially: Obsidian Cliff, a wall of black volcanic glass. Obsidian makes razor-sharp tools. Archaeologists have traced Yellowstone obsidian to sites hundreds of kilometres away, which shows that this stone moved along long trade routes.

The outside world learned about Yellowstone partly through pictures. In 1871, a government survey crossed the region. With it travelled the photographer William Henry Jackson and the painter Thomas Moran. Earlier explorers had described boiling rivers and exploding fountains, and many people refused to believe them. Jackson's photographs and Moran's paintings helped convince lawmakers that these wonders were real. The park was created the following year.

The animals are the other half of the story. Yellowstone is the only place in the United States where bison have lived continuously since prehistoric times. You will often see them on the roads, moving slowly and without concern for traffic. In 1995, grey wolves were brought back to the park after decades of absence. Their return changed how elk moved and grazed, and scientists still study the effects across the whole landscape.

Water shapes the park in quieter ways too. The Yellowstone River carved a deep canyon with walls of yellow, pink and cream rock. Its Lower Falls drop more than twice the height of Niagara Falls. In the north, at Mammoth Hot Springs, hot water carries dissolved limestone to the surface. The mineral hardens into white terraces that look like frozen waterfalls, and they change shape year after year.

A word about the ground itself. The crust around the hot springs can be thin, and the water beneath can be hot enough to cause severe burns. This is why the boardwalks exist. They protect you, and they also protect the fragile mineral formations and microbial mats. Stay on them, even when a view looks better from the side. Animals here are wild too. A bison can run much faster than a person.

Before you move on, pause and look around you. Notice the steam rising and fading. Notice the colour of the ground, the sound of water moving underneath, the small birds that come to drink at the edges. Everything here is in motion, slowly or suddenly. When you walk away, listen once more for that low gurgle. The earth under Yellowstone is still at work — and it will keep working long after you leave.