Beneath the Tuscan countryside that powers the oldest geothermal energy grid on Earth, seismic imaging has now mapped more than 5,000 cubic kilometres of molten rock sitting less than 10 kilometres below the surface, a volume on the scale of the magma reservoirs that produced the largest eruptions in human prehistory. Findings published in Communications Earth & Environment in April 2026 quantify the full extent of the magmatic system underneath the Larderello-Travale geothermal field in southern Tuscany, Italy, placing it in direct comparison with Yellowstone, Toba, and the Long Valley caldera. The region has supplied geothermal electricity since 1904 and sits within a landscape of medieval hilltowns, vineyard estates, and a permanent population with no active volcano warning infrastructure in place, because until now, no one classified this as a volcanic system at all.
The mechanism that makes this so alarming is temperature and pressure, not geological age. At 2.8 kilometres depth in the Larderello field, a borehole drilled as part of the DESCRAMBLE project recorded 512 degrees Celsius and 42.5 megapascals of pressure. At those conditions, water does not exist as liquid or steam in the conventional sense; it crosses into a supercritical state, behaving simultaneously like a gas and a liquid, and carrying far more thermal energy per unit volume than ordinary steam. The geothermal gradient in this zone exceeds 150 degrees Celsius per kilometre, meaning temperature climbs roughly five times faster with depth than the continental crust average of around 30 degrees per kilometre. That extreme gradient cannot be sustained by residual heat from ancient geological processes; it requires an active, ongoing heat source operating at depth right now.
That heat source is what the new seismic survey has resolved for the first time at full scale. A network of 63 broadband seismometers, deployed across southern Tuscany from September 2020 to September 2021 and integrated with Italy’s national INGV monitoring grid, recorded continuous ambient ground vibration. Those vibrations, processed using a technique called ambient noise tomography, allowed scientists to reconstruct three-dimensional shear-wave velocity maps of the crust from the surface down to 15 kilometres depth. Shear waves, the secondary seismic waves produced by earthquakes and ground movement, slow dramatically when they pass through molten or partially molten material, because liquids cannot sustain the shearing motion that gives these waves their name. The maps showed shear-wave velocities dropping to 1.25 kilometres per second at 10 kilometres depth below Larderello, a reduction of approximately 40 percent below the regional baseline.
A 40 percent reduction in shear-wave velocity is not a subtle signal. Comparable drops have been mapped beneath the Toba caldera in Indonesia, beneath Yellowstone in Wyoming, and beneath the Long Valley caldera in California, all systems that have produced eruptions exceeding 600 cubic kilometres of volcanic material. To translate what those velocities mean physically, the team used thermodynamic modelling to calculate melt fractions: the proportion of the rock that is currently liquid versus crystalline. The core of the anomaly below Larderello carries a melt fraction above 80 percent, meaning it is predominantly liquid magma rather than solid rock with melt interspersed. That 80-percent liquid core spans approximately 3,000 cubic kilometres. Surrounding it sits a crystal-rich mush shell, material that is partly solidified but still contains around 20 percent liquid, adding roughly another 5,000 cubic kilometres to the total volume. Below Mt. Amiata to the southeast, where the most recent eruptions in this region occurred around 300,000 years ago, the volumes appear at least twice as large, though the edges of that anomaly reach the limits of the survey’s resolution and require further verification.
To contextualise that volume: the Long Valley caldera in California, which produced the Bishop Tuff eruption approximately 760,000 years ago, has a mapped crustal reservoir of around 6,400 cubic kilometres. Yellowstone’s full magmatic system, traced from the mantle plume to the upper crust, spans roughly 10,000 cubic kilometres. The Larderello reservoir at 5,000 cubic kilometres sits in the same order of magnitude as the lower end of that range, making it directly comparable in scale to recognised supervolcano plumbing systems. The heat flow at Larderello, which peaks at 1,000 milliwatts per square metre, matches recorded heat flow at Yellowstone, where 2,000 milliwatts per square metre has been measured across the caldera. At Larderello, the surface shows no caldera, no volcanic cone, no recognised eruption in the Holocene, which is the geological epoch covering the last 11,700 years.
That absence of a volcanic surface structure is what kept this system off the threat register for so long. Geologists classify a system as volcanic when there is at least one documented eruption and a recognisable surface feature such as a cone or caldera. The Larderello field has neither. What it has is identical geophysical signatures: intense shallow seismicity, gravity anomalies centred over the magma body, vigorous hydrothermal fluid flow, fumarolic venting that was so intense before industrial exploitation began that the area was historically named Devil’s Valley, and a heat flow rivalling the most active volcanic systems on the planet. The only metric missing from the standard volcanic checklist is an eruption, and the reason for that absence appears to lie in the chemistry of the magma itself.
The magma stored beneath the Tuscan Magmatic Province is classified as peraluminous anatectic granite, a highly viscous type of melt generated by the partial melting of the sedimentary basement rocks of the Tuscan crust rather than by mantle-derived basalt ascending from below. Because the viscosity of magma scales exponentially with silica content and with the proportion of aluminium-rich minerals, this material is orders of magnitude more resistant to flow than the basaltic magmas that drive eruptions at Toba or Yellowstone. Highly viscous magma cannot ascend efficiently through the crust; it tends to pond, spread laterally, and cool in place, forming underground granite bodies called plutons rather than erupting at the surface. The same viscosity barrier that makes eruption unlikely is also what allows so much material to accumulate in one place at shallow depths without producing the surface morphology that would normally trigger geological alarm.
That pluton-forming process has physically lifted the land surface above the system by approximately 500 metres since the Pliocene, which is the geological epoch that ended around 2.6 million years ago. Vertical uplift at that scale over that timeframe is consistent with the emplacement of large volumes of buoyant crustal material at depth. Current ground deformation above the Larderello field records 0.13 millimetres of uplift per year, a rate that is low compared with actively inflating calderas like Campi Flegrei near Naples, which has measured approximately 1 metre of uplift per year during its most recent unrest episodes. The slow rate of current deformation does not indicate that the system is passive; it indicates that the magma is not currently intruding at the rate that would precede an imminent eruption. The distinction matters because this system has operated at extreme heat flow and supercritical fluid conditions continuously, without acceleration, for the duration of historical record.
The fluid dynamics above the magma body carry their own infrastructure-scale consequences. The supercritical fluids that pool at around 3 kilometres depth above the magma core are the direct energy source for the Larderello geothermal field, which feeds electricity into the Italian national grid. Those fluids transfer heat from a reservoir that is, by the measurements now in hand, vastly larger than any previous survey had resolved. The K-horizon, a geologically debated reflective boundary detected by seismic surveys since the 1980s at roughly 3 to 4 kilometres depth, now maps as the ceiling of the fluid zone overlying the magma, not a tectonic boundary or an independent geological feature. Below that ceiling, temperatures exceed 500 degrees Celsius. Above it, the geothermal infrastructure operates. The vertical separation between commercial drilling operations and supercritical magmatic fluid conditions is less than one kilometre in some parts of the field.
The seismic survey also identified that the low-velocity anomaly below Larderello is not an isolated pocket but part of a regionally continuous magmatic zone extending southeast to the Piancastagnaio and Amiata geothermal systems across a horizontal distance of more than 100 kilometres. That lateral extent places multiple geothermal operating zones, multiple provincial towns, and large sections of southern Tuscany over a single connected magmatic province of supervolcano-scale dimensions. No eruption history exists for this province in the past 300,000 years, and no surface structure marks it as a recognised volcanic hazard zone under current Italian geological risk classification. The survey’s seismic network is now decommissioned, having completed its data collection period in September 2021. No permanent dense seismic monitoring array equivalent to those deployed at Yellowstone or Campi Flegrei currently operates across the full extent of the Tuscan Magmatic Province.
Source:
Lupi, M. et al. High-enthalpy Larderello geothermal system, Italy, powered by thousands of cubic kilometres of mid-crustal magma. Communications Earth & Environment 7, 269 (2026). DOI: https://doi.org/10.1038/s43247-026-03334-0
