Far up in the north of California, at the southern edge of the Cascade Mountains, lies one of the state’s biggest—and steamiest—hidden treasures. This is Lassen Volcanic National Park, one hundred and sixty-five square miles of crystal clear lakes, striking mountain views, and bubbling geothermal activity.

Lassen is special for many reasons, but particularly rare is the fact that here, you can visit all four types of volcanos: plug dome, shield, cinder cone, and composite. It’s one of the few places in the world—and the only park in the United States—with these specific bragging rights. But don’t worry: while the park has technically been volcanically active for about three million years, its peaks are currently dormant, and pose no risk to visitors. Instead, they’re a great way to learn about how geothermal activity presents itself on the surface of the earth.

A Pile of Rocks

The great thing about Lassen’s volcanoes is that you can hike every type. For park ranger Paola Hinojosa, the most satisfying hike is the cinder cone volcano, aptly named: Cinder Cone.

Cinder cone volcanoes are “like a popcorn popper,” Hinojosa explains: the volcano ejects gaseous lava particles, which turn to rock when they hit the air and quickly cool. These rocks, who owe their bubbly exteriors to the gas trapped inside them, pile up to create a circular volcanic cone. So, while it may look like solid ground from far away, Cinder Cone is actually a very dense pile of rocks. Hiking it is a “task of perseverance,” Hinojosa says, but worth it. “It’s like going up a sand dune—every step you take forward isn’t as productive as you’d like it to be. But it’s an otherworldly place, with views of Lassen peak and the surrounding lakes.”

Surrounding Lassen peak is a “Devastated Area” full of rocks flung out from its last eruption.
Surrounding Lassen peak is a “Devastated Area” full of rocks flung out from its last eruption. Alex Frankel / Atlas Obscura

That Namesake Peak

Lassen Peak towers over the park. It’s over 10,000 feet tall, and an example of a plug dome, or lava dome, volcano. These volcanoes are defined by their thick, viscous lava, which piles up and creates a steep dome-like shape. Lassen last erupted over a period of three years, just over 100 years ago: 1914 to 1917. In May of 1915, an explosive eruption caused a mud flow: the hot rocks erupting from the volcano mixed with snow, creating an avalanche-like flow of mud that destroyed a patch of surrounding land. This is now called the Devastated Area, which includes a gentle trail, a few enormous rocks that were once expelled from the nearby peak, and signs that tell you the story of the 1915 explosion.

A view of Lassen from Brokeoff Mountain.
A view of Lassen from Brokeoff Mountain. Alex Frankel / Atlas Obscura

An Invisible Volcano

Ranger Paola’s favorite volcano in the park is the composite volcano, but it’s not the easiest to find. “When people come to the Visitors Center, I always tell them, ‘You’re in the middle of a volcano!’” The Kohm Yah-mah-nee Visitor Center lies at the middle of what was once Mount Tehama, a composite volcano that towered over Lassen Peak at 11,500 feet and a gargantuan thirteen miles wide. It has eroded over time, and now only parts of it remain.

“Volcanic rock is pretty fragile and susceptible to the harsh elements of snow and rain gnawing at it,” ranger Paola explains. “Glaciers in the last glacial period shaped the volcanoes, so you’ll find glacial striations of where the glaciers would rub up against the rock, almost like sandpaper.” Around Brokeoff Mountain—what remains of this enormous, eroded volcano—you’ll see what look like claw marks, scratches left by ice.

Brokeoff Mountain offers some of the park’s most stunning views.
Brokeoff Mountain offers some of the park’s most stunning views. Alex Frankel / Atlas Obscura

Brokeoff mountain also offers one of Lassen’s most beautiful hikes, seven miles that take you through meadows and forests, past lakes and up 2,600 feet of elevation. “On a clear day,” ranger Paola says, “you can see Lassen peak. You’re eye to eye with the beast, and you see just how glorious and impressive it is.”

Don’t Forget Your Shield

The fourth and final volcano, a shield volcano, is flatter and wider than their imposing brethren. Lassen’s shield volcano, Prospect Peak, was formed by layers and layers of thin lava flows that build up like layers of paint on a canvas.

A watched mud pot always boils.
A watched mud pot always boils. Alex Frankel / Atlas Obscura

Why’s That Mud Boiling?

Volcanoes aren’t the only sites for geothermal exploration in Lassen. Hike up to the Bumpass Hell mud pots, and you’ll find patches of mud that looks like the bubbling surface of a pot of soup—but smells like rotten eggs.

These mud pots are the result of rain and snowfall in the park. That water seeps into the ground through permeable rock, trickling six to nine miles down towards large bodies of magma. When the magma heats the water, it creates steam that travels back up towards the surface, and escapes as water vapor. At the Bumpass Hell mud pots, that vapor heats the mud, causing it to bubble up and steam.

When you approach these mud pots, or the nearby Sulfur Works—another site of geothermal activity where steam escapes from vents in the ground—you might notice the smell of rotten eggs. This smell is transported up from that deep-down magma, not someone’s overcooked lunch.

“The rotten egg smell comes from hydrogen sulfide gas,” explains park ranger Paola Hinojosa. “It’s a volcanic product, and when the hydrogen sulfide reaches the surface, it reacts with oxygen and forms sulfur crystals.” You can sometimes spot these bright yellow crystals at the edge of hydrothermal areas, like Bumpass Hell.

Steam escapes from the ground at one of Lassen’s many fumaroles.
Steam escapes from the ground at one of Lassen’s many fumaroles. Alex Frankel / Atlas Obscura

Fuming, But Not Mad

The final type of geothermal activity you’ll find in the park is a large number of fumaroles, or steam vents. From far away, these look like trains of smoke that might be coming out of someone’s cabin set along one of the park’s peaks. In fact, fumaroles are slits in volcanoes, where volcanic gasses and vapors creep out towards the sky.

Even in the snow, Lassen’s lakes are majestic.
Even in the snow, Lassen’s lakes are majestic. Alex Frankel / Atlas Obscura

Magma Is Cool, But Don’t Miss The Water

While Lassen is a fascinating place to learn about volcanoes and volcanic activity, there are hundreds of other reasons to visit the park. If you’re feeling tired or have limited mobility, take a scenic drive on Lassen Volcanic National Park Highway, where you’ll get views of the park’s peaks and wild landscape, and stop-offs at some of its geothermal sites. There are also a wide number of trails that take you past the many mountain lakes that appear throughout Lassen’s peaks, with crystal clear (and very cold!) mountain water. The Cluster Lakes Loop is a 13-mile hike that will take you past 12 different lakes. It’s a long route, and best done with an overnight stay in the park. Lassen is great for backcountry hiking, especially because of the incredible stargazing available on clear nights. After a few days at Lassen, your mind might start thinking that the sky is full of shimmering magma.