The steam has been coming out of the ground for as long as anyone has lived in the region.
Farmers in central Tuscany noticed it centuries ago — thin plumes of white vapour rising directly from cracks in the earth, hissing quietly, smelling faintly of sulphur. The medieval Italians who named the area picked the obvious explanation. This was Valle del Diavolo. Devil’s Valley. The Earth was releasing something it was not supposed to be releasing.
Prince Piero Ginori Conti was the first person to work out that the steam could be used to generate electricity. In 1904, he ran a small demonstration turbine on Larderello’s natural steam that lit four light bulbs. By 1911, the world’s first industrial geothermal power plant was operating on the site — a facility that has been generating electricity, in various forms, for over a hundred years since.
For most of that century, nobody could fully explain where all the heat was coming from.
The missing volcano
The Larderello region has all the geological signatures of a serious volcanic system. Intense shallow earthquake activity. Gravity anomalies. Vigorous hydrothermal circulation. Heat flow rivalling the most active volcanic regions on Earth.
What it does not have is a volcano.
There is no cone. There is no caldera. There is no eruption recorded in either the historical or the geological record of the past several hundred thousand years. Every conventional criterion for classifying a system as volcanic requires at least one documented eruption and at least one recognisable surface feature. Larderello has neither.
This absence is what kept the system quietly off the international volcanic-hazard register for over a century. Geologists knew something was producing the heat. Boreholes drilled at the site had recorded sudden temperature spikes at depths of a few kilometres. But without a surface feature to point at, and without any of the standard imaging techniques finding anything definitive, the specific source remained a matter of educated inference rather than direct observation.
That changed in April 2026.
What the seismic wave map revealed
A research team led by Matteo Lupi at the University of Geneva, working with Italian colleagues at the INGV and CNR-IGG, deployed more than 60 seismometers across Tuscany and its nearby islands. They then used a technique called ambient noise tomography — a relatively new method that turns the background hum of the Earth into a map of the subsurface — to look at what was actually below the region.
The map that emerged contained a substantial anomaly. Seismic waves travelling through a specific region beneath Larderello were moving markedly slower than through the surrounding rock. Slow seismic velocity is a well-established signature of partially molten material. When the team calculated the volume of the anomaly, the number they arrived at was extraordinary.
Approximately 6,000 cubic kilometres of magma, sitting between roughly 8 and 15 kilometres below the surface, spread across a region about 20 kilometres wide.
For context, Lake Geneva contains approximately 89 cubic kilometres of water. The Larderello magma reservoir contains, by volume, more than sixty times as much molten rock as Lake Geneva contains water. It is in the same size class as the magma reservoirs beneath Yellowstone, Toba in Indonesia, and Long Valley in California — the largest volcanic systems currently known on Earth.
The team’s paper, published in Communications Earth & Environment in April 2026, is the first direct quantification of what had, until then, been an educated inference.
Why it does not erupt
The natural question is why a reservoir of that size, that shallow, has not produced a catastrophic eruption at some point in the region’s history.
The answer appears to lie in the specific chemistry of the magma. Unlike the magma beneath Yellowstone, which is a silica-rich rhyolite that has periodically erupted in enormous explosions, the material beneath Larderello has been identified as peraluminous anatectic granite. This is a specific class of magma produced when sedimentary rocks in the middle crust partially melt — a process geologists call anatexis.
Anatectic granite magma is highly viscous. It moves extremely slowly. It does not readily ascend to the surface, and it does not efficiently accumulate the dissolved gas pressure required to drive an explosive eruption. In essence, the material beneath Larderello is too thick and too gas-poor to behave the way the magma beneath Yellowstone behaves.
The structure of the overlying crust also appears to play a role. The specific geological configuration of the region — the layers of sedimentary rock, the tectonic faulting patterns, the geometry of the crust itself — seems to have functioned as a natural containment vessel, trapping the magma at depth rather than allowing it to migrate upward.
So the reservoir has been quietly heating the region for a very long time without doing anything dramatic. The steam vents at the surface are the visible portion. The magma stays where it is.
What this changes about geothermal science
The practical significance of the finding extends well beyond Larderello itself.
Larderello has been producing geothermal electricity for over a hundred years. Until now, the specific mechanism sustaining that output has been understood in general terms but not quantified. The new map explains why the site has been so productive for so long — the reservoir is enormous, it is close to the surface, and it has been continuously supplying heat to the overlying rock and water at rates that make sustained industrial exploitation viable.
It also suggests that similar systems may exist elsewhere. Ambient noise tomography, the technique used to find the Larderello reservoir, is relatively cheap and can in principle be applied anywhere in the world where there is enough seismic activity to produce usable background signal. Which is essentially everywhere. Lupi’s team has noted that the same method could be used to identify other hidden magma bodies that might power currently unrecognised geothermal resources — as well as, they cautioned, other systems that have been quietly sitting below populated regions without being properly characterised.
What the reservoir is doing right now
The magma beneath Tuscany has been there for a very long time.
Zircon dating of rocks at the surface, which formed from previous magmatic episodes in the region’s history, suggests the underlying magmatic system has been active for at least the last five million years. It has probably been intermittently supplying the current reservoir over that entire span. It is still doing so today.
The reservoir is not currently in a state that suggests imminent eruption. Its chemistry works against explosive behaviour, and its containment appears stable. But the researchers who mapped it are careful to point out that “not currently” is not the same as “never” — and that a system on this scale, sitting relatively shallow beneath a densely populated region of Europe, is worth monitoring more carefully than it has been.
The steam continues to rise from Devil’s Valley. It has been rising, in various forms, for as long as anyone has lived nearby. It is powered by something that most people who have visited the region, driven past it, or eaten dinner in one of its restaurants, have never known was there. It is not going anywhere soon. It has, quietly, been sitting under Tuscany for millions of years, patiently doing what magma reservoirs do — producing heat, driving convective circulation in the rock above, and reminding anyone paying attention that the surface of the planet is thinner than it looks.
