No one invented geothermal energy. It is natural heat stored beneath the Earth’s surface, coming partly from the slow breakdown of radioactive particles in rocks. People have used this resource for thousands of years, long before the word “geothermal” existed. The history of geothermal energy is a story of gradual discovery: from ancient civilisations bathing in hot springs to modern power plants generating grid electricity.
Today, geothermal energy provides a steady, round-the-clock source of power in specific regions of the world. It has evolved from simple bathing and heating into an industrial technology that generates electricity and, at one Cornish site, extracts lithium. This article traces that journey, correcting common historical errors and looking at where the technology stands now. For the basics, see our guide to geothermal energy and how it works.
How it works
Geothermal energy starts with heat from deep inside the Earth. This heat moves toward the surface through conduction and convection. In some areas, it brings water close to the surface naturally, creating hot springs or geysers. In other places, the rock is hot but dry, requiring engineers to drill deep wells to create artificial reservoirs.
The technology used to harness this heat depends on what is available underground. High-temperature steam or hot water can drive turbines to generate electricity. Lower-temperature water, often found closer to the surface, is piped directly into buildings for heating. Ground-source heat pumps use the stable temperature of shallow ground to heat and cool homes; the first was built in 1948 by Carl Nielsen of Ohio State University for his own house. While the source is always the same, the methods of extraction have become increasingly precise.
Real-world examples
The timeline of geothermal use is often misunderstood. Historical records show a progression from simple thermal use to industrial application, and finally to electricity generation.
Ancient bathing and heating
The Romans are frequently credited with using geothermal energy, but specific examples are often misquoted. The Baths of Caracalla in Rome, inaugurated in AD 216, did not use geothermal water. They were heated by wood-fired furnaces feeding a hypocaust system. A better example is the Roman settlement at Bath, England (Aquae Sulis), which had three hot springs. According to the Roman Baths museum, the Temple of Sulis Minerva dates to the later first century AD, and the spring water emerges at 46 °C. The springs flow at more than a million litres a day. The Romans built a temple and baths around them, using the water for bathing and worship.
Centuries later, in the 14th century, wooden pipes in the French town of Chaudes-Aigues carried hot spring water to about 30 houses. This is often cited as the world’s oldest geothermal district heating system.
The birth of geothermal industry and electricity
The first industrial use of geothermal energy was not for power, but for chemicals. In 1818, François Jacques de Larderel established a plant near Montecerboli in Tuscany, Italy, which used geothermal fluids to extract boric acid. Enel, which now runs the Tuscan geothermal fields, dates the start of the geothermal industry to this plant.
The leap to electricity came decades later. In 1904, Prince Piero Ginori Conti used geothermal steam from Larderello to drive a dynamo that lit five light bulbs. This was the first geothermal electricity. In 1913, the first commercial geothermal power station was installed at Larderello, with a 250 kW turbo-alternator. The plants there were destroyed late in the Second World War and rebuilt. According to ThinkGeoEnergy (2024), Enel operates 34 geothermal plants in Tuscany, supplying 34% of the region’s electricity needs.
Early American and global developments
In the United States, commercial use began later. In 1830, Asa Thompson charged visitors one dollar each to use three spring-fed baths in Hot Springs, Arkansas. A US Department of Energy timeline records this as the first known commercial use of geothermal energy in the US. In 1892, Boise, Idaho, built a district heating system using water piped from hot springs. It was one of the first such systems in the world, and the first in the US.
In Iceland, Reykjavík began municipal geothermal district heating in 1930. Reykjavík Energy’s own history records that hot water from boreholes at the Laugardalur springs was piped about three kilometres to a local school in November that year.
The second commercial geothermal power station in the world, after Larderello, was in New Zealand. At Wairakei, drilling began in 1949, and the first generator was commissioned on 15 November 1958. Engineering New Zealand describes it as the first to use flash steam from geothermal water.
The first commercial geothermal power station in the US followed at The Geysers in California, where PG&E’s Unit 1 began commercial operation on 25 September 1960. It had 11 MW of net capacity and was built for less than $2 million.
Enhanced Geothermal Systems (EGS)
Because natural high-temperature reservoirs exist in only a few places, engineers developed Enhanced Geothermal Systems (EGS), which create reservoirs in hot dry rock. The US Department of Energy calls Fenton Hill in New Mexico the first successful EGS project in the world. Running from 1971 to 1995, it drilled eight major wells and created two reservoirs in crystalline rock at depths of 2.8–3.5 km. It produced 3–10 MW thermal, demonstrating that the concept was feasible.
The UK followed with the Hot Dry Rock programme at Rosemanowes Quarry in Cornwall from 1977 to 1994. Three wells were drilled to 2.6 km in the Carnmenellis granite, reaching temperatures of about 100 °C. This was a rock-mechanics experiment, never meant to generate electricity.
More recently, Fervo Energy synchronised its Cape Station project in Utah to the grid on 24 September 2026. It declared commercial operation for its first 33 MW block on 30 September 2026. The company calls this the first greenfield enhanced geothermal development in the world to achieve contractual commercial operations. A further 400 MW is expected in 2028.
What the numbers say
Geothermal energy is a mature industry with steady growth. According to IRENA, world geothermal power capacity was 15,624 MW at the end of 2025, up from 12,169 MW in 2016. ThinkGeoEnergy reported a higher figure of 17,173 MW across 35 countries. The two sources differ mainly on the US: IRENA lists it at 2,696 MW, while ThinkGeoEnergy lists it at 3,953 MW.
IRENA puts global geothermal generation at about 97.7 TWh in 2024. The IEA (2024) says geothermal meets about 1% of global electricity demand. The average global capacity factor was 88% in 2024, meaning the average plant ran close to full output for most of the year.
On IRENA’s end-2025 figures, Indonesia leads in installed capacity with 2,744 MW, followed by the USA at 2,696 MW, the Philippines at 2,057 MW, Türkiye at 1,758 MW and New Zealand at 1,283 MW. In Iceland, 90% of housing is heated directly with geothermal energy, and geothermal supplied 62% of the country’s primary energy in 2018.
In the UK, the history is quieter but recently active. The Southampton District Energy Scheme started in 1986 or 1987 (sources differ), producing about 1.7 MW thermal from a single well; it was reported closed for repairs in June 2023, and its current status is unclear. The Eden Project in Cornwall began using deep geothermal heat in June 2023 from a well 4,871 metres deep. The United Downs project, near Redruth, saw drilling between November 2018 and June 2019. Its production well reached a measured depth of 5,275 metres, the deepest in the UK. First power was announced on 26 February 2026 from a 3 MWe gross ORC plant, and lithium carbonate production at the site also began in February 2026.
Limitations / honest assessment
Geothermal energy is not a universal solution. It is location-dependent. High-temperature resources suitable for electricity generation are concentrated along tectonic plate boundaries and in volcanic areas. Elsewhere, including the UK, reaching useful temperatures means drilling deep: the Rosemanowes wells found about 100 °C at 2.6 km, and the United Downs production well goes to 5,275 metres.
Reservoir management is a critical challenge. The Geysers in California grew to well over 1,000 MW by the late 1980s, but output fell as reservoir pressure declined. Treated wastewater injection began in 1997 to recharge the reservoir. Without careful management, geothermal fields can lose pressure and output.
The upfront cost of drilling is high, and there is geological risk: a well that finds too little heat or permeability can lose its capital. Engineering problems have stalled projects too. The first attempts at The Geysers in the early 1920s were abandoned because pipes and turbines could not cope with the abrasive, corrosive steam, and finance was lacking. EGS technologies promise to unlock heat in more locations, but they are only now reaching commercial operation.
The UK angle
The UK’s geothermal history has been driven by research rather than commercial success. The Southampton scheme is described by the British Geological Survey as the only tangible legacy of the UK Geothermal Energy Programme of the 1970s and 1980s. After the Rosemanowes experiment closed in 1994, the UK saw a long gap: the Eden Project well was the country’s first operational deep geothermal well since 1986.
The United Downs project marked a turning point. Drilling concluded in mid-2019 and, after testing and stimulation between 2018 and 2021, the plant switched on in February 2026. Its developer, Geothermal Engineering Ltd, calls this the UK’s first-ever geothermal electricity, and you can read more about the UK’s first geothermal power plant in Cornwall. The project holds a Contract for Difference for 2 MW, awarded in 2023, and it also produces lithium. The Gateshead mine water heat scheme, which began full operation on 29 March 2023 with 6 MW of heat pumps fed by water from 150 m below the town, shows another avenue: using abandoned mines to provide heat. For how geothermal fits alongside other sources, see the live GB electricity mix.
Frequently asked questions
Who discovered geothermal energy, and when was it first used?
No single person discovered geothermal energy. It is a natural phenomenon. People have used hot springs for bathing and heating for thousands of years; a US Department of Energy timeline says Paleo-Indians used them more than 10,000 years ago. A well-documented early example is the Roman temple and baths at Bath, dating to the later first century AD.
When was geothermal energy first used to make electricity?
Geothermal electricity was first generated in 1904. Prince Piero Ginori Conti used steam from Larderello, Italy, to light five light bulbs. The first commercial geothermal power station followed in 1913, also at Larderello.
Where was the first geothermal power plant in the UK?
At United Downs, near Redruth in Cornwall. It began generating power in February 2026, and its developer calls this the UK’s first-ever geothermal electricity. The earlier Hot Dry Rock programme at Rosemanowes (1977–1994) was an experiment that was never meant to generate electricity.
Did the Romans use geothermal energy in Britain?
Yes. The Romans built a temple and baths at Aquae Sulis (modern-day Bath) around hot springs whose water emerges at 46 °C and flows at more than a million litres a day. The famous Baths of Caracalla in Rome, by contrast, were heated by wood-fired furnaces, not geothermal water.
How has geothermal energy changed over time?
It has moved from simple thermal use (bathing, heating) to industrial chemical extraction (boric acid from 1818), then to the first electricity (1904) and commercial power stations (from 1913), and more recently to enhanced geothermal systems. At United Downs, lithium is now extracted alongside power generation.
Which country uses the most geothermal energy today?
On IRENA’s figures, Indonesia has the largest installed geothermal power capacity, with 2,744 MW at the end of 2025, followed by the USA with 2,696 MW and the Philippines with 2,057 MW. ThinkGeoEnergy counts US capacity higher, at 3,953 MW. For heating, Iceland stands out, with 90% of its housing heated directly by geothermal energy.