Geothermal Energy
The Planet Has Been Running a Nuclear Reactor for 4.5 Billion Years — We're Just Starting to Tap It
Beneath your feet, radioactive decay is generating more heat than the entire human civilisation currently consumes — and we've barely figured out how to use it.
The Idea
The Earth is not a cold rock that happened to stay warm. It is an actively heat-generating system, powered by two overlapping engines. The first is primordial: residual thermal energy left over from the violent collisions that assembled the planet. The second is nuclear: the slow decay of radioactive isotopes — primarily uranium, thorium, and potassium-40 — locked inside the mantle and crust. Together, these sources push roughly 47 terawatts of heat continuously toward the surface. That is about twice what humanity currently uses in all forms of energy combined. The challenge is not the supply. It is the gradient. Heat flows usefully only when there is a temperature difference between a hot source and a cooler sink — a basic thermodynamic constraint. At the surface, that difference is modest. You have to drill deep enough to reach genuinely useful temperatures, and in most places, the heat is spread too thinly to be tapped economically. But geology is not uniform. In volcanic zones — Iceland, Kenya's Rift Valley, parts of New Zealand and the western United States — the mantle pushes close enough to the surface that steam or superheated water becomes accessible within a few kilometres of drilling. These are the places where geothermal has quietly worked as baseload power for decades. The more exciting frontier is Enhanced Geothermal Systems, or EGS: a technique that essentially creates an artificial geothermal reservoir by fracturing hot dry rock, injecting water, and extracting the heat on demand — potentially unlocking the vast regions where geology is less cooperative.
In the World
In 1970, Iceland was still burning imported oil to heat its homes. Today, roughly 90 percent of the country's buildings are heated by geothermal water piped directly from the ground, and nearly a third of its electricity comes from geothermal turbines. Reykjavik sits on the Mid-Atlantic Ridge, where two tectonic plates pull apart and the mantle wells upward — geology so extreme that you can walk between the plates in a valley outside the city. The Hellisheiði power plant, one of the largest geothermal facilities in the world, produces both electricity and hot water for the capital, all from a field of wells drilled into volcanic rock. What makes Iceland worth studying is not just the resource — it is the institutional decision to use it. In the mid-twentieth century, the government funded the drilling of test wells, absorbed early losses, and built out a district heating network before the economics were obviously compelling. The result is that Icelanders now pay some of the lowest heating costs in Europe and emit almost nothing from domestic warmth. The more recent and globally significant story is what is happening in the Salton Sea region of California, where a nascent EGS industry is trying to prove that geothermal can work far from volcanic hotspots. Fervo Energy drilled a horizontal well pair in Nevada in 2023, circulated water between them through fractured granite, and demonstrated sustained power generation — a proof-of-concept that the technology can reach beyond the lucky latitudes.
Why It Matters
Most clean energy conversations orbit solar and wind, and reasonably so — their costs have collapsed dramatically. But both are intermittent, and the harder problem of a fully decarbonised grid is not generating electrons cheaply on sunny, windy days; it is keeping the lights on when conditions are still and dark. Geothermal is one of the few renewable sources that runs continuously, day and night, regardless of weather. If Enhanced Geothermal Systems prove out at scale, they could offer firm baseload power from a resource that is effectively inexhaustible on any civilisational timescale, available in most parts of the world, and with a land footprint far smaller than solar or wind farms of equivalent output. The broader reframe worth carrying is this: we tend to think of energy as something we must manufacture or harvest from the sky. But the planet itself is a heat engine, and the ground beneath any city on Earth is warmer the deeper you go. The question EGS is really testing is whether the thermodynamic gradient beneath ordinary rock — not just Iceland, not just Yellowstone — is close enough to economic reality to change what the global energy system looks like by mid-century.
A Question to Ponder
If geothermal energy is technically available almost everywhere on Earth, what does it tell us about how we choose which energy sources to develop — and who gets to make that choice?
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