Hydrothermal Areas

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Learn where and how to find hydrothermal or "hot water" areas at Lassen Volcanic National Park.

The remarkable hydrothermal features in Lassen Volcanic National Park include roaring fumaroles (steam and volcanic-gas vents), thumping mud pots, boiling pools, and steaming ground. Water from rain and snow that falls on the highlands of the park feed the hydrothermal system. Once deep underground, the water is heated by a body of hot or molten rock beneath Lassen Peak. Rising hot water boils to form boiling pools and mud pots. Super-heated steam reaches the surface through fractures in the earth to form fumaroles such as those found at Bumpass Hell and Sulphur Works. These features are related to active volcanism and are indications of the ongoing potential for further eruptions from the Lassen Volcanic Center.

View images of Lassen hydrothermal areas at npgallery.nps.gov/lavo. All images are public domain and available for fair use.
Hole in ground at Sulphur Works where a visitor stepped off-trail and broke through.
A visitor stepped on thin ground hiding boiling water and mud at Sulphur Works

NPS Photo

Hydrothermal Area Safety

For your safety, stay on established trails and boardwalks. Ground in hydrothermal areas can look solid but may actually be a thin crust hiding pools of acidic boiling water or mud. Traveling off-trail in these areas may result in severe injury.

Visitors Have Been Injured When Traveling Off-Trail.

  • A visitor was severely burned in the summer of 2010 after he traveled off-trail in the Devils Kitchen hydrothermal area. He stated that "It feels like I put my leg in a flame."
  • On May 5, 2012, a visitor was air-lifted to a regional burn unit after stepping off the sidewalk at Sulphur Works. The ground appeared solid, but she easily broke through a one-inch crust, exposing her foot and ankle to boiling acidic water and mud.

Where to Find Them

Bumpass Hell

Note: The boardwalk and portions of the trail are closed for a multi-year rehabilitation project. Access the largest hydrothermal area in the park via a 3-mile round-trip hike. A boardwalk takes visitors through a 16-acre bowl of plopping mudpots, bubbling pools, and roaring steam vents - including the super hot Big Boiler.

Little Hot Springs Valley

Located at the bottom of a steep valley, steam vents can be viewed via the park road with binoculars. There is no trail in this area.

Pilot Pinnacle

There is no trail or parking area for this feature which includes steam vents, boiling pools and mudpots. One part of this area is visible from the park road; "Fart Gulch" is a chalk-colored hillside on the north side of the road near Little Hot Springs Valley. The sulfur smells makes this area easily identifiable.

Sulphur Works

The park's most easily accessed hydrothermal area features boiling mudpots and steam vents viewable via a sidewalk. A parking area with sidewalk access to the features is located one mile north of the Southwest Entrance.

Devils Kitchen

This moderate hiking trail in the Warner Valley area leads visitors to this bubbling cauldron. Explore steam vents, mudpots, and boiling pools on a short loop.

Boiling Springs Lake

Accessed from the Warner Valley trailhead, this short hike leads to a bubbling lake with a temperature of the lake around 125 degrees. Mudpots and steam vents line part of the shore and drainage creeks. Be careful to stay on clearly marked trails in this area as the ground around the lake is unstable; travel in these areas may result in severe injury.

Terminal Geyser

Access this gigantic steam vent from the Warner Valley trailhead. Although not a true geyser, this spurting steam located in the middle of a creek, provides a spectacular show!

Cold Boiling Lake

Enjoy a short hike from the Kings Creek Picnic Area to this quaint lake where "cold boiling" bubbles rise like soda water.


Last updated: April 23, 2018

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