Geothermal energy is a renewable resource that harnesses heat from within the Earth to generate electricity and provide heating. The Earth's core, composed of molten rock and metal, reaches temperatures exceeding 5,000 degrees Celsius. This immense heat conducts outward through layers of rock, warming underground reservoirs of water and steam. Geothermal power plants tap into these reservoirs by drilling wells up to several kilometres deep. The hot water or steam is then brought to the surface, where it drives turbines connected to generators. Unlike solar or wind power, geothermal energy is available continuously, regardless of weather or time of day, making it a reliable baseload power source.

There are three main types of geothermal power plants: dry steam, flash steam, and binary cycle. Dry steam plants use steam directly from underground vents to spin turbines; they are the oldest type, first used in Larderello, Italy, in 1904. Flash steam plants are the most common today; they pull high-pressure hot water into lower-pressure tanks, causing it to 'flash' into steam that drives a turbine. Binary cycle plants pass moderately hot water through a heat exchanger, where it heats a secondary fluid with a lower boiling point; that fluid vaporises and turns the turbine.

Binary plants can operate at lower temperatures, expanding the number of viable sites. Geothermal resources are not evenly distributed around the world. The most accessible reservoirs lie along tectonic plate boundaries, where volcanic activity and crustal fractures bring heat closer to the surface. Countries such as Iceland, the Philippines, Indonesia, and New Zealand have abundant geothermal energy because they sit on the Pacific Ring of Fire. Iceland, for instance, generates about 25% of its electricity from geothermal sources and uses geothermal heat for nearly 90% of its homes. In contrast, regions far from plate boundaries, like much of Australia, have fewer high-temperature reservoirs, though enhanced geothermal systems (EGS) are being developed to extract heat from deeper, less permeable rock.

Binary cycle plants pass moderately hot water through a heat exchanger, where it heats a secondary fluid with a lower boiling point; that fluid vaporises and turns the turbine.

Enhanced geothermal systems involve fracturing hot, dry rock deep underground to create artificial reservoirs. Water is injected through one well, flows through the fractured rock, absorbs heat, and is then pumped up a second well to the surface. This technology could vastly expand geothermal potential, but it faces challenges: induced seismicity (small earthquakes), high upfront drilling costs, and the need for large volumes of water. Despite these hurdles, EGS projects are underway in countries like the United States, Australia, and Japan. If successful, they could unlock geothermal energy in areas previously considered unsuitable.

Geothermal energy has significant environmental advantages. It emits far fewer greenhouse gases than fossil fuels—binary plants release almost no carbon dioxide. Land use is minimal compared to solar farms or wind parks; a geothermal plant typically occupies about 1–8 acres per megawatt of capacity. However, there are drawbacks. Geothermal fluids often contain dissolved minerals and gases, such as hydrogen sulfide, which can be released during operation. Reinjecting the fluids back into the reservoir helps reduce emissions and maintain pressure, but it requires careful management to avoid contamination of groundwater.

Additionally, geothermal plants can cause land subsidence if too much fluid is extracted without reinjection. The cost of geothermal electricity has declined over the past decade, but it remains higher than that of solar or wind in many regions. Drilling exploration wells is expensive and carries a risk of failure—up to 20% of exploratory wells do not find a viable reservoir. Once a plant is built, however, operating costs are low because the fuel (heat) is free. The levelised cost of electricity (LCOE) for geothermal ranges from about $50 to $100 per megawatt-hour, depending on resource quality and plant type.

Government incentives and carbon pricing can improve its competitiveness. As technology improves and drilling techniques become more efficient, geothermal is expected to become more affordable. Looking ahead, geothermal energy could play a larger role in the global energy mix. The International Renewable Energy Agency (IRENA) estimates that installed geothermal capacity could reach 28 gigawatts by 2030, up from about 14 gigawatts in 2020. Innovations such as closed-loop systems, which circulate a working fluid through a sealed pipe deep underground without extracting water, could reduce environmental risks and allow development in more locations. Geothermal also offers the possibility of direct use for district heating, greenhouse agriculture, and industrial processes. With continued research and investment, the heat beneath our feet may become an increasingly important part of a sustainable energy future.