The first geothermal generator lit just four light bulbs in 1904
Humans have bathed in hot springs since the Stone Age, and Romans charged admission to the steaming pools at Bath. Turning Earth's heat into electricity came much later, when an Italian prince lit four bulbs at a Tuscan steam field. That same site still produces hundreds of megawatts today.
Geothermal energy is heat drawn from Earth's crust, the name built from Greek words for earth and hot. Our planet holds an immense store of internal heat, about a fifth of it left over from its formation and the rest from the decay of radioactive elements. A borehole 5,275 metres deep in Cornwall, for instance, struck granite rich in thorium whose decay keeps the rock hot. Near the boundary between core and mantle, temperatures top 4,000 degrees Celsius.
People put this warmth to work long before electricity. The oldest known spa is at the Huaqing Chi palace, and when Romans took Aquae Sulis, today's Bath, in the first century they used its springs for public baths and underfloor heating; the entry fees were probably the first commercial use of geothermal energy. Chaudes-Aigues in France has run a geothermal district heating network since the fifteenth century. Industry arrived in 1827 at Larderello in Italy, where geyser steam helped extract boric acid from volcanic mud, and Boise, Idaho, heated homes this way from 1892.
Larderello also produced the first power. On 4 July 1904 Prince Piero Ginori Conti tested a generator there, and in 1911 the world's first commercial geothermal station followed, remaining the only one until New Zealand opened a plant in 1958. The first American plant started at The Geysers in California in 1960, and its original turbine ran for more than 30 years. Binary cycle stations, first shown in the Soviet Union in 1967, can work with much cooler water; one in Chena Hot Springs, Alaska, generated electricity from water at just 57 degrees Celsius in 2006.
Geothermal plants run steadily whatever the weather, and costs fell by a quarter during the 1980s and 1990s. The US Department of Energy put new-plant power at about five cents per kilowatt-hour in 2021. Even so, the 16 gigawatts available worldwide in 2025 made up less than one percent of renewable capacity.
Source: Geothermal energy
