Roughly 500,000 people live inside the caldera of Campi Flegrei, the supervolcano just west of Naples, and for years they have felt the ground rise and shake beneath them. Between 2020 and 2024, more than 4,000 earthquakes rattled the area, feeding fears that magma was pushing toward the surface. But a growing body of peer-reviewed research now points to a different engine: a shallow pocket of superheated water and gas, trapped 2 to 4 kilometers underground, is generating the pressure behind the quakes and ground uplift, not molten rock.
Why superheated fluids, not magma, change the risk calculus for Naples
The distinction between rising lava and pressurized hydrothermal fluids is not academic for the communities sitting on top of Campi Flegrei. Magma moving upward would signal a possible eruption timeline. A gas-rich reservoir behaves differently: it can swell, crack overlying rock, and release energy in seismic swarms without any eruption at all, yet it can also weaken the caprock that holds it in place. A 2025 study published in Nature Communications used high-resolution imaging and rock-physics constraints to conclude that deformation and seismicity at Campi Flegrei are governed by a shallow, gas-rich, pressurized reservoir and fluid migration, and that the results exclude the presence of magma in the first 4.5 km of depth.
That finding reframes how officials and residents should think about the hazard. If the energy source is condensing fluids rather than ascending magma, the threat is not a classic volcanic eruption but repeated cycles of ground uplift, cracking, and sudden pressure release. Research comparing distinct unrest periods at Campi Flegrei has identified microstructural evidence that hydrothermal water can promote caprock sealing, meaning the lid above the reservoir can tighten between episodes. A tighter seal allows pressure to build faster during the next pulse of hot fluid, which raises a practical concern: the interval between major pressure-release events could be shortening compared with earlier unrest cycles. Researchers have not yet confirmed that acceleration with time-series data, but the physical mechanism for it is documented.
For residents, this translates into a chronic, uneven risk. Even if no eruption occurs, uplift and seismic swarms can damage buildings, strain infrastructure, and erode public confidence in official messaging. Civil protection agencies must therefore plan not only for low-probability, high-impact eruptions, but also for recurring, disruptive unrest that may demand evacuations or building reinforcements without a clear eruptive endpoint.
Seismic imaging and CO2 data trace the buried reservoir
The clearest picture of what sits beneath Campi Flegrei comes from tomographic analysis of 4,161 earthquakes recorded between 2020 and 2024. Researchers used large phase-pick and differential-time datasets to build a 3D velocity and Vp/Vs structure down to roughly 4 km depth. Specific velocity patterns they identified are consistent with pressurized processes and upward fluid migration within the shallow system, not with a body of molten rock.
Separate geochemical work has linked the hydrothermal system’s pressure and temperature changes to measured CO2 emissions and seismicity at the surface. That study, published in the Journal of Volcanology and Geothermal Research, provided quantitative energy-scale comparisons showing that thermal energy released by deep condensation of fluids dwarfs the cumulative seismic energy recorded during unrest episodes. In plain terms, the heat released when superheated water condenses underground is more than enough to account for the shaking residents feel, without invoking magma.
Competing models have not disappeared entirely. INGV researchers and other volcanologists have debated whether a hydrothermal “sponge” pressurization model or a magma-intrusion model better explains the data. Reporting by Nature’s climate and environment desk has documented that tension, noting that the two hypotheses carry very different implications for civil protection planning. The sponge model suggests ongoing, cyclical hazard from fluid pressure. The magma model implies a more binary risk: either an eruption is approaching or it is not.
The weight of recent tomographic and geochemical evidence has shifted toward the fluid-pressure explanation. Multiple independent datasets, from seismic velocity inversions to gas emission measurements, converge on the same confined reservoir between 2 and 4 km depth. No study in the current literature has identified a magma body within that range. Instead, the crust appears to be pervaded by a hot, gas-rich network of fractures and pores that can transmit pressure efficiently across the caldera.
Open questions about caprock integrity and pressure timing
Several gaps remain in the scientific record. The raw differential-time datasets and full phase-pick catalogs behind the 4,161-earthquake tomography have not been released in formats that allow independent replication outside the summarized results. A multi-year CO2 emission time series integrated with the Vp/Vs velocity models has not been published as a single unified dataset, which limits the ability to track how gas output and subsurface structure co-evolve in real time.
The caprock-sealing hypothesis, while physically supported by microstructural evidence, lacks direct measurement of sealing rates during the current unrest episode. Researchers have shown that hydrothermal fluids can deposit minerals that tighten fractures in overlying rock, but they do not yet know how quickly this happens at Campi Flegrei today, or how sealing competes with ongoing cracking from earthquakes. That balance will determine whether the system trends toward more frequent, smaller pressure releases or rarer, larger ones.
Another unknown is how far the shallow hydrothermal reservoir is hydraulically connected to deeper magmatic sources. Even if no molten body is present within the top 4.5 km, deeper magma can still supply heat and volatiles. Resolving that connection will require joint inversion of seismic, gravity, and magnetotelluric data, paired with high-frequency gas monitoring at fumaroles and wells. Such integrated datasets are technically demanding and expensive to acquire, and they require sustained institutional support.
Implications for monitoring and civil protection
For authorities in Naples and the surrounding towns, the emerging hydrothermal picture argues for a monitoring strategy that emphasizes fluid pathways as much as magma movement. That means denser seismic networks tuned to detect subtle changes in Vp/Vs ratios, continuous CO2 and sulfur gas measurements at key vents, and satellite-based ground deformation tracking that can distinguish broad uplift from localized fault motion.
It also underscores the importance of transparent communication. Residents have lived through multiple unrest cycles since the 1980s, including evacuations that were not followed by eruptions. Explaining that current shaking is likely driven by pressurized fluids, not an imminent magmatic breakout, can reduce panic-but only if paired with clear statements about residual risks, such as the possibility of phreatic explosions or damaging earthquakes without lava.
On the research side, scientists are turning to large, curated databases to cross-check models and quickly share results. Profiles of key contributors, such as volcanologist Chiara Sabelli, and broader repositories like NCBI-hosted archives illustrate how open data and code sharing can accelerate understanding of complex systems like Campi Flegrei. Extending that openness to seismic and gas-monitoring records from the caldera would help international teams test competing models more rigorously.
For now, the consensus emerging from recent work is cautiously reassuring: there is no evidence of a large, shallow magma body poised to erupt beneath Naples. Yet the same studies warn that a volatile, superheated hydrothermal system can still deliver damaging earthquakes, ground deformation, and localized explosions. Living atop Campi Flegrei will remain a negotiation with an active, pressurized subsurface-and the more precisely scientists can image and monitor that hidden reservoir, the better prepared its half-million residents will be for whatever the next unrest cycle brings.
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*This article was researched with the help of AI, with human editors creating the final content.
