The Ground Won't Sit Still

The field in brief

Volcanology at a restless caldera is less about lava than about reading a buried machine through its symptoms at the surface. A caldera is the broad basin left when a volcano empties its magma chamber in a huge eruption and the roof collapses into the void; Campi Flegrei, the Phlegraean Fields, is one such basin roughly 12 to 15 kilometers across, built by two cataclysms about 39,000 and 15,000 years ago.[5] Its defining behavior is bradyseism, the slow rise and fall of the ground as pressure below builds and eases, from the Greek for slow movement.[6] The town of Pozzuoli sits at the center of the bowl, and its Roman market columns still carry bands of marine borings that record centuries of sinking and lifting.[5]

Two more ideas do most of the work in the fight. A hydrothermal system is the zone of hot groundwater and steam that sits above a magma body, charged with gases that rise from the melt; at Campi Flegrei it vents through the Solfatara crater and the Pisciarelli fumaroles, which together release four to five thousand tonnes of carbon dioxide a day, enough to rank the caldera among the strongest volcanic gas sources on Earth.[3] The crucial distinction is between magmatic unrest, driven by molten rock physically moving toward the surface, and hydrothermal or phreatic unrest, driven by that overlying water-and-gas system heating and pressurizing without any fresh magma arriving.[5] The two can look nearly identical from above, and telling them apart is the discipline's hardest forecasting problem.

The last concept is how rock breaks. Under modest stress the crust behaves elastically, flexing and springing back; push harder and it enters an inelastic regime, where cracks grow, link, and do not heal, until the rock approaches bulk failure and ruptures.[2] Small brittle-failure earthquakes, the sharp snaps of rock fracturing at shallow depth, are the audible edge of that process. Whether the crust here is merely flexing or genuinely tearing is the center of the dispute.

The fight

Campi Flegrei has erupted only once since Roman times, throwing up the small cone of Monte Nuovo in 1538 after months of ground swelling.[5] The modern unrest arrived in pulses: a rise of about 74 centimeters in 1950 to 1952, 159 centimeters in 1969 to 1972, and 175 centimeters in 1982 to 1984, when the port district of Pozzuoli lurched upward, swarms of earthquakes cracked buildings, and some 25,000 people were evacuated.[1] Then the ground sank back, and the caldera went quiet. Around 2005 it began rising again, slowly at first, and it has not stopped.

The new episode has been gentler in rate but stubborn in duration, lifting the center of the bowl more than 1.5 meters since 2005 and, at its recent peaks, climbing faster than two centimeters a month.[6] The seismicity sharpened. A magnitude 4.4 shock in May 2024 became the strongest to strike the area in some forty years; a magnitude 4.6 offshore in June 2025 then took that title.[7][6] Italy's Civil Protection Department has held the caldera at a yellow alert, the second of four levels, meaning attention and heightened monitoring rather than imminent danger, since 2012.[6]

Into this rising ground, in June 2023, came the paper that framed the current fight. Christopher Kilburn of University College London, with Stefano Carlino, Stefania Danesi, and Nicola Alessandro Pino of Italy's National Institute of Geophysics and Volcanology, reconstructed seventy years of quakes and uplift and argued that the crust had crossed from elastic flexing into the inelastic regime around 2020.[1] By their reading the rock is now roughly two-and-a-half to three times weaker in tension than it was in 1984, damaged by decades of stretching, and each centimeter of fresh uplift now buys far more earthquakes than it once did, the signature of a material approaching failure.[1] They placed the driving pressure below the hydrothermal system, in deep gas, and treated the surface gas flux as a symptom of uplift rather than its cause.[1]

Kilburn's group was careful about what this does and does not mean. Rupture is not eruption; a completed break could open a path for magma, or the crust could simply relax and subside as it has before.[1] But the reframing unsettled colleagues who read the same signals differently. Giovanni Chiodini and co-workers have long argued that the unrest is chiefly the work of fluids: magmatic gases rising from depth into the shallow hydrothermal system, heating it, pressurizing it, and dissolving the rock's own carbonate, without requiring fresh magma to intrude at shallow levels.[3] In this view the swelling and the tremors are the hydrothermal system straining, and reading them as a countdown to rupture claims more than the data can bear.

A third position holds that magma is in fact moving. Modeling of the deformation since 2005 has been read as a shallowing, widening source consistent with intrusions at three to five kilometers, and a 2025 forecast led by Luca Caricchi of the University of Geneva, with Carlino among the authors, treated each unrest pulse as a magma injection and concluded that the reservoir would still need two to three decades of feeding before it could drive an eruption on its own.[4] Yet a machine-learning analysis of the seismicity the same year found no evidence of magma rising above about 3.7 kilometers, tilting back toward the fluid picture.[5] The stakes hold the argument taut. Roughly half a million people live inside the caldera, and the Red Zone evacuation plan governs who moves first; a false alarm empties a city needlessly, and a missed one is unthinkable.[5]

What the fight reveals

The dispute exposes how thin the ground is beneath caldera forecasting. No borehole reaches the shallow source, which sits kilometers down, so every camp is inferring an unseen machine from the same surface twitches, and the disciplines diverge because a physicist reading rock strength, a geochemist reading gas, and a modeler reading deformation are each looking through a different window.[5] The historical record offers little comfort: of the five unrest episodes since 1950, only the distant 1538 event actually erupted, while calm calderas elsewhere have both erupted with little warning and swelled for decades without ever breaking.[5]

On imminence, the evidence leans against panic. Every camp, including Kilburn's, stops short of forecasting an eruption soon, and the alert has stayed yellow through the strongest shaking in a generation.[1][6] Kilburn's narrower claim, that the crust has entered a weakening, inelastic regime, has held up as a description of the seismic and deformation record, and the appearance since 2022 of more coherent earthquake mechanisms tracing a ring fracture is consistent with a crust that is breaking rather than merely flexing.[4] What that breaking leads to remains open.

The mechanism itself is genuinely unresolved. Whether the source four to five kilometers down is intruding magma or pressurized hydrothermal fluid is still argued in the current literature, and the same 2024 and 2025 datasets are cited by both sides.[5] That uncertainty is not a failure of effort but a property of the system, and it is the uncomfortable core of the decision facing Naples: the crust is measurably straining toward something, and the science cannot yet say whether that something is an eruption or another slow exhale.