The Geysers, California: The World's Largest Geothermal Field

By Geothermal Insider · Updated September 10, 2026

Despite the name, there is no geyser at The Geysers. Calpine, which has been the field's dominant operator for a quarter century, says so on its own history page. In 1847, a hunter named William Bell Elliott was tracking a grizzly bear through the mountains north of San Francisco when he came across what Calpine's timeline calls the "Gates to Hades." He named the place The Geysers, "though in fact there are no geysers here." Calpine puts it bluntly: "Although there are no actual geysers at this wonder-inspiring region, the inaccurate name stuck."

What Elliott found were fumaroles and hot springs — steam and hot water venting from fractured rock, not the periodic eruptions that define a true geyser. The name has outlived the mistake by nearly 180 years and now sits on the largest geothermal power complex in the world.

Its production history is just as unusual. This field peaked in the late 1980s, then declined — not because the heat ran out, but because the steam did. Since 1997, operators have replenished the reservoir by piping treated municipal wastewater into the mountains and injecting it underground.

What The Geysers Is, and Where

The Geysers is a geothermal steam field in the Mayacamas Mountains of Northern California, roughly 120 kilometers north of San Francisco according to USGS Circular 1249. The California Energy Commission describes it as "the world's largest geothermal field," located "in Sonoma, Lake, and Mendocino counties."

Calpine, the operator, draws the county lines slightly differently. Its own site labels the complex as 28,447 "Acres (45 square miles) in Sonoma and Lake Counties and bordering Mendocino County" — its 2025 statistics sheet rounds the same acreage to 44.5 square miles. The CEC is describing the geological field; Calpine is describing its own 28,447-acre operating complex.

The 45-square-mile complex holds multiple plants, linked by steam pipelines running over the ridgelines. Calpine states that it "owns and operates 13 power plants at The Geysers" and calls the complex "the largest complex of geothermal power plants in the world." It has been a wholly owned subsidiary of Constellation Energy since January 2026, when Constellation completed its acquisition; Calpine remains the operating name on the ground.

Land ownership underneath is a patchwork. A 2011 Stanford paper by Sanyal and Enedy describes the field as a checkerboard of federal, state and private lease blocks, without giving a percentage split.

Why It's Called The Geysers When There Are None

A geyser is a hot spring that erupts periodically, throwing water and steam into the air on something close to a schedule. Old Faithful is a geyser. The features at The Geysers are fumaroles — vents that release steam and gas — along with hot springs that sit and steam rather than erupt.

Elliott's 1847 misnaming was understandable from a distance. Steam rising from a hillside in dozens of places looks, to someone who has not seen a real geyser field, like one.

Tourism then locked the name in. From 1860 to 1880, according to Calpine's timeline, "The Geysers is in its prime as a major tourist destination, with famous visitors from around the world." U.S. Boundary Commissioner John Bartlett had already created what Calpine calls "the earliest known illustrations" in 1852. Once a place is on stagecoach itineraries under a particular name, geologists are not going to correct it.

The Geology: How You Get a Steam Field

The Geysers is unusual because it is a dry-steam field; most exploited geothermal fields began as hot-water reservoirs. More than 90% of exploited geothermal fields were liquid-dominated before production began — hot pressurized water rather than steam, which either flashes to steam at the surface or, at lower temperatures, heats a separate working fluid in a binary plant. A 2009 paper in Geothermics reports the proportion: "More than 90% of exploited fields are 'liquid-dominated' under pre-exploitation conditions." The Geysers is in the other category. It is vapour-dominated, a dry-steam field, and dry steam is the simplest resource in geothermal because you can point it straight at a turbine. The California Energy Commission explains: "In the Geysers Geothermal area, dry steam from below ground is used directly in the steam turbines."

Calpine describes a million-year transition from hot-water reservoir to steam field:

"A million years ago, a plume of molten magma intruded close to the Earth's surface. The heat from this 1400˚F intrusion recrystallized the overlying rocks, making them hard and brittle, and causing them to fracture creating permeability. Subsequent magmatic activity over the next half-million years maintained high temperatures as water seeped down through fractures to form a hot water geothermal reservoir. About a quarter million years ago, the caprock over the ancestral Geysers reservoir fractured. As high-temperature water boiled away, steam eruptions followed, creating the steam reservoir that remains today."

When the caprock fractured, much of the reservoir water boiled away, leaving steam in the fracture network. A 1973 USGS paper describes the result: "steam dominates the large fractures and voids of the reservoir and is the continuous, pressure-controlling phase."

USGS describes the likely source as a "silicic magma chamber more than 7 km below the earth's surface" — an interpretation it flags as resting on geophysical studies of limited resolution, and in part controversial, rather than a settled measurement. Reservoir temperatures in the main vapor-dominated reservoir, per USGS Open-File Report 95-30, "generally are in the range 238°C to 244°C"; the same report notes a deeper reservoir in the northwestern field above 315°C.

Its steam can therefore drive turbines directly, with no flashing stage and no separate working fluid. Our guide to the three types of geothermal power plants covers how dry steam, flash and binary designs differ, and why the resource dictates the choice.

From Resort to Power Plant: The Timeline

Calpine's timeline starts 12,000 years ago: "Paleoindians arrive at The Geysers. They are the earliest-known human inhabitants of the region." Of the Native peoples who followed, the timeline says "The tribes share access to The Geysers, which they use for healing and ceremony." Commercial development began in the 1920s, with John Grant and two wells.

1921–1934: The first attempt, and its failure

Grant's first drilling attempt in 1921 ended in a blowout. His second, in 1922, was "only 200 feet deep" and was "the first successful geothermal well at The Geysers." In 1923 the family organized The Geysers Development Company and built a 35-kilowatt power plant — "generating the first geothermal electricity in the Americas." Its 35-kilowatt output was roughly the combined draw of a dozen homes.

The experiment ended in 1934, when cheap oil made geothermal power uncompetitive.

1955–1968: The commercial era begins

Magma Power Company drilled Magma 1, "the first modern geothermal well at The Geysers," in 1955. In 1958, Pacific Gas and Electric agreed to buy steam from Magma-Thermal — the contract that made a power plant financeable. In 1960, PG&E "completes Unit 1, the first commercial geothermal electric unit in the Western Hemisphere, generating 11 megawatts of electricity."

By 1968, "electrical generation capacity at The Geysers reaches 82 megawatts." Most of the field's generating capacity was added through the 1970s and 1980s.

1989–2009: Consolidation and stabilization

Calpine's own entry into the field was tiny: a "one-megawatt interest in the 20-megawatt Aidlin power plant" — 1989 on its own timeline, October 1988 in an SEC-filed company presentation. Eleven years later it bought the field. Its 2000 entry reads: "Calpine acquires 19 Geysers power plants and associated steamfields. Consolidated ownership eliminates contractual barriers, allowing a fieldwide approach."

A fieldwide approach let one operator manage reservoir pressure across a shared steam system. A steam reservoir is a shared resource: when a dozen companies each produce against the same underground pressure, the incentive to conserve pressure is weaker than it would be under one owner.

By 2009, per the timeline: "With power plant upgrades and new sources of water to produce steam, Calpine's electrical generation at The Geysers stabilizes and has remained so." Those "new sources of water" are the two wastewater pipelines.

When The Geysers Peaked

The Geysers reached peak output, peak installed capacity and peak steam production at slightly different times.

Peak electrical output came in 1987. Dobson and colleagues, writing in Geothermics in 2020, put it at a "peak output of ~1.6 GWe in 1987."

Peak installed capacity was 2,043 MW. Calpine's timeline dates that to 1987 as well: "Production peaks at The Geysers. Twenty-one power plants are in operation, with a total installed capacity of 2,043 megawatts." The National Research Council, in a 2013 publication, dates the same figure to 1989 — "installed generation capacity was increased to a total of 2,043 MW through building 28 additional power plant turbine-generating units."

Peak steam production is dated differently again. USGS Circular 1249 states flatly: "Steam production peaked in 1988, and has declined since then."

The dates differ because output, installed capacity and steam production measure different parts of the field, and would not necessarily peak in the same year. And 2,043 MW is installed capacity — hardware in the ground, not what the field produced; the output figure for the same era is Dobson's ~1.6 GWe.

The apparent conflict over 2,043 MW comes from how the dates are described. Calpine identifies the peak year as 1987; the National Research Council describes the longer build-out reaching that figure through 1989. Installed capacity reached 2,043 MW in the late 1980s.

The Wastewater Fix

The decline came from a loss of reservoir fluid, not from cooling rock. The rock did not cool down. As Truesdell put it in a paper archived by OSTI: "Pressure and flow declines at The Geysers are recognized as due to decrease in reservoir fluid rather than decrease in reservoir temperature or heat content."

Steam was being withdrawn faster than nature replaced the water that made it. The heat source kilometres down was not going anywhere; what was running short was something to boil. So the operators went looking for water, and found it in treated municipal wastewater — from the ten communities served by Lake County's system, and separately from the City of Santa Rosa.

Lake County: the SEGEP pipeline, 1997

In 1995, Calpine, NCPA and Lake County Sanitation agreed to build the Southeast Geysers Effluent Pipeline — a 29-mile line to deliver treated wastewater into the mountains. Lake County records the dedication as October 16, 1997, and the total construction cost at $45 million.

Calpine's history gives the design volume as 9 million gallons a day. Lake County's current page reports actual delivery as "an average of 5,400 Gallons Per Minute," roughly 7.8 million gallons a day. The county attributes a measurable result to it: "These industry partners have achieved a 70 MW increase in generating capacity since Phase 1 operations began."

Santa Rosa: the 40-mile line, 2003

The second pipeline was bigger. In 2003, Calpine and the City of Santa Rosa completed the Santa Rosa Geysers Recharge Project — a 40-mile pipeline on Calpine's timeline, 41 miles on the city's current project page — "bringing an average of 11 million gallons a day." Santa Rosa describes it as "a $220 million investment by Santa Rosa and its regional partners" and credits the project with about 100 megawatts of additional power. (The city's own fact sheet phrases this as "generated each day", which mixes up power and energy — megawatts measure the rate, not a daily total.) The city and Calpine marked 20 years of the partnership in 2023.

Calpine's timeline notes that the pipeline route partly follows an 1877 road built to serve the mercury boomtown of Pine Flat.

Between them, the two pipelines pump the wastewater of Santa Rosa and Lake County uphill into a steam reservoir, where it boils and comes back as the steam that turns the turbines. Sewage is a working input to the world's largest geothermal field. It also shows that a geothermal reservoir can be drawn down faster than it naturally recharges, then replenished through injection. We cover that argument in whether geothermal energy is actually renewable.

How Big Is It Now?

Current estimates vary by owner, by what is being measured and by publication date.

Capacity. Calpine puts its own 13 plants at "up to 725 megawatts," and its 2020 Sustainability Report labels that figure in capitals: "725 MW (NET GENERATION CAPACITY)." Net generation capacity here means the maximum the plants can deliver under specified conditions after their own station use; it is neither the turbines' nameplate rating nor what the field actually generated in any given year, which is measured in megawatt-hours. That 725 MW also predates the expansion below, and Calpine has not published a reconciled post-expansion total for its own fleet.

Output. Calpine's own current account of the field says "Today The Geysers generates approximately 850 MW of renewable, clean, reliable, base load power." For comparison, Dobson and colleagues wrote in 2020 that "current production levels for the entire field are around 750-800 MWe." Both are approximate snapshots of power rather than annual measurements, so they do not establish a trend.

Plant count. Calpine's "By the Numbers" sheet, labelled 2024 statistics, reports: "there are 15 geothermal power plants operating at The Geysers and Calpine Corporation, the largest geothermal power producer in the U.S., owns and operates 13 power plants." The other two are the Northern California Power Agency's Sonoma County Units 1 and 2 — NCPA is a separate owner-operator in the same field, as it said in a January 2023 release: "NCPA and Geysers Power Company, LLC both individually own and operate geothermal plants located in The Geysers area." A USGS page updated in September 2023 still says "There are 18 geothermal plants," which "produce about 835 megawatts of electricity"; that number is three years old, and the California Energy Commission lists at least one former plant, Bottle Rock, as non-operational.

For statewide context, the California Energy Commission's table "Installed In-State Electric Generation Capacity by Fuel Type (MW)" puts geothermal at 2,715 MW in 2025, flat from 2,715 in 2024 and 2023, up from 2,709 in 2022 and 2,664 in 2021. Those figures cover all California geothermal — the Salton Sea, Coso and Mammoth are in there alongside The Geysers — so it cannot be read as this field's capacity, though The Geysers is part of the total. The same commission's generation table, "In-State Electric Generation by Fuel Type (GWh)," shows statewide geothermal at 10,908 GWh in 2025, 10,453 in 2024, 10,999 in 2023, 11,123 in 2022 and 11,116 in 2021: a resource holding roughly level while the grid changes shape around it. Our California geothermal guide covers the wider state picture.

Earthquakes at The Geysers

The Geysers area is seismically active, and its activity is linked to what happens in the wells. USGS, on a page updated August 14, 2025, states that "increased steam production and fluid injection correlates positively with changes in earthquake activity."

Recorded earthquakes there have stayed moderate in magnitude. USGS answers it directly: "To date, the largest quake recorded at The Geysers is approximately M4.5." On the outlook, the agency says "It is possible that a magnitude 5 could occur, but larger earthquakes are thought to be unlikely." On the trend, "The level of seismicity has been fairly stable since the mid-1980s." And on depth, USGS places them shallower than 6 km; early monitoring in Open-File Report 78-798 found them "confined to depths of less than 5 km."

Recent Projects at The Geysers

North Geysers Incremental Development. Calpine completed this 25 MW expansion in 2026 — "We've completed a 25 MW expansion at The Geysers, enough to power ~25,000 California homes" — with 18 MW going to Clean Power Alliance and the 7 MW first phase already serving MCE. It is the first meaningful new geothermal capacity built at the field in years.

Battery storage. In 2024 Calpine added two Battery Energy Storage systems totaling 38 megawatts at the West Ford Flat and Bear Canyon sites.

Enhanced geothermal. A completed Department of Energy demonstration at The Geysers, with $6 million in DOE funding, "successfully yielded enough steam to produce 5.8 MW of electricity."

Closed-loop. GreenFire Energy ran a closed-loop test in a legacy well with $2,705,228 in California Energy Commission funding; the CEC's April 2026 final report describes a completed 90-day operational test. Over a 30-day segment at 330 gallons per minute it sustained "7.4 megawatts of thermal power," modeling to 0.9 MW gross electric, and peaked near 9 MW thermal for a modeled "around 1.1 megawatts of electric power" gross. The thermal figures measure heat output; the modeled electric figures measure gross electrical output.

Visiting The Geysers

The steamfield is closed to the public, but the visitor center and scheduled tours are open.

The field itself is closed. Calpine's 2025 facility road map is labeled "NO PUBLIC ACCESS." The power plants and steamfield are an industrial site on private and leased land, and you cannot drive in and look around.

The visitor center is open and free. Calpine's Geothermal Visitor Center in Middletown, California "is open to the public, free of charge."

Tours exist, and they book out. Calpine "offers free community tours on a preset, annual schedule," and "Reservations are required." Calpine's own published notice marks the 2026 season FULLY BOOKED, with the instruction to "Check back here for our 2027 tour schedule on November 13, 2026."

For a 2027 tour, check the operator's visitor center page when the schedule posts on November 13, 2026.

Frequently Asked Questions

Are there actually geysers at The Geysers?

No. Calpine's history page states it plainly: William Bell Elliott named the place in 1847 "though in fact there are no geysers here," and "the inaccurate name stuck." The features are fumaroles and hot springs.

Where exactly is The Geysers?

In the Mayacamas Mountains of Northern California, about 120 kilometers north of San Francisco per USGS. Calpine describes the complex as 28,447 acres — 45 square miles — in Sonoma and Lake Counties and bordering Mendocino County. The California Energy Commission describes the field as being in Sonoma, Lake and Mendocino counties.

How much power does The Geysers produce?

Calpine says the field "generates approximately 850 MW" today, across 15 operating plants of which Calpine owns and operates 13 at "up to 725 megawatts." For comparison, Dobson and colleagues estimated whole-field production around 750–800 MWe in 2020, and a USGS page updated in September 2023 says the field's 18 plants "produce about 835 megawatts of electricity."

Why did production decline after the 1980s?

Steam was withdrawn faster than water recharged the reservoir, which dropped pressure. Truesdell states that declines were "due to decrease in reservoir fluid rather than decrease in reservoir temperature or heat content."

Does The Geysers really run on wastewater?

Two pipelines inject treated municipal wastewater into the reservoir: a 29-mile line from Lake County dedicated in 1997, and a 40-mile line from Santa Rosa completed in 2003 that has pumped about 11 million gallons a day since. Lake County reports a 70 MW increase in generating capacity from its project; Santa Rosa credits its Recharge Project with about 100 megawatts of additional power.

Are the earthquakes at The Geysers dangerous?

USGS says the largest quake recorded there is approximately M4.5, that a magnitude 5 is possible but larger events "are thought to be unlikely," that the level of seismicity "has been fairly stable since the mid-1980s," and that the quakes are shallow — USGS places them above 6 km depth.

Can I visit The Geysers?

You can visit the free Geothermal Visitor Center in Middletown. The field itself is marked no public access. Free tours require reservations; the 2026 season is fully booked and the 2027 schedule is due to post on November 13, 2026.

Who owns The Geysers?

Calpine owns and operates 13 plants there and has been a wholly owned Constellation Energy subsidiary since Constellation completed its acquisition in January 2026. NCPA separately owns and operates geothermal plants in the same field.

What makes The Geysers different from other geothermal fields?

It is a dry-steam, vapour-dominated field, which is rare — a 2009 Geothermics paper notes that "more than 90% of exploited fields are 'liquid-dominated' under pre-exploitation conditions." Dry steam can go straight to the turbine. For how the other resource types are converted to electricity, see geothermal power plants; for the basics of where the heat comes from, start with what geothermal energy is, and for other major fields around the world, see geothermal energy examples.