The Volcano that Breathes and Heals Itself: Campi Flegrei's Recurring Unrest

Imagine living in a town where the ground slowly rises and falls as if the Earth itself were breathing. Streets lift up by a meter, only to sink back down over the span of years. This eerie phenomenon has a name – bradyseism, meaning "slow earthquake". It's not a violent shake but a gentle, restless heaving of the land. One place on Earth is famous for it: the Campi Flegrei caldera in southern Italy. For centuries, observers have been perplexed by the caldera’s odd habit of swelling and deflating, puzzling over what hidden forces could be at play.

Campi Flegrei sits just west of Naples, in a region aptly called the Phlegraean Fields – the "fiery fields" of Roman lore. Unlike a classic cone-shaped volcano, Campi Flegrei is a broad, sunken volcanic bowl, or caldera, formed by ancient colossal eruptions. It’s a landscape of steaming fumaroles, boiling mud pools, and yellow sulfur stains – clear signs that heat and gas still lurk below. And yet, what grabs everyone’s attention is how the very ground underfoot behaves like a giant diaphragm: steadily bulging upward for years (sometimes alarmingly fast), then eventually settling back down. Why does this volcano seem to breathe in and out? Recent research offers a fascinating explanation involving rainwater, trapped steam, and even self-healing rocks.

Caldera Unrest and the Mystery of Bradyseism

First, let’s break down what's happening during one of Campi Flegrei’s “breathing” episodes. Caldera unrest is a general term for when a dormant volcano shows signs of waking up – like increased seismic tremors, ground deformation, or gas emissions. Many calderas around the world go through ups and downs, but Campi Flegrei’s style is special. The Italian term bradisismo (from which bradyseism derives) was coined after scientists noticed the slow rising and sinking of land near the coastal town of Pozzuoli, on the caldera’s edge. In fact, if you visit the ruins of an ancient Roman marketplace in Pozzuoli (misnamed the “Temple of Serapis”), you can see bands of boreholes left by marine mollusks on its columns – evidence that the land went down below sea level long ago and then rose back up, lifting those columns above water again.

Bradyseism, as observed at Campi Flegrei, is essentially slow-motion uplift or subsidence of the ground. Think of the ground swelling as a kind of inflation – like a balloon being slowly inflated deep below the surface. The subsidence (sinking back) is like the balloon later deflating. Importantly, these changes can happen without any lava erupting at the surface. They often come with swarms of small earthquakes and an extra whiff of sulfur from the volcano’s gases. For decades, scientists suspected that something like magma intrusion – fresh molten rock pushing its way in – might be inflating the caldera like a balloon. But Campi Flegrei’s unrest has some peculiar traits that weren’t fully explained by magma alone. This led researchers to dig deeper into the volcano’s plumbing.

Inside Campi Flegrei: A Geologic Pressure Cooker

To understand Campi Flegrei’s behavior, you need to picture its internal structure. Beneath the picturesque towns and vineyards lies a hydrothermal system – essentially, hot fluids (water and gases) circulating through cracked rock. There is indeed magma way down (roughly 7–8 kilometers deep, scientists think), which acts like a giant stove, heating everything above it. But between the magma and the surface, Campi Flegrei has layers of rock and ash deposits from past eruptions. Notably, about 1–2 kilometers below ground is a layer of tough, cemented volcanic ash and tuff from a massive ancient eruption (~39,000 years ago). This layer serves as a caprock, a hard lid trapping fluids underneath. Beneath this lid, at around 2–4 kilometers depth, sits a confined reservoir of hot, pressurized fluid – a mix of groundwater, rainwater, and volcanic gases like carbon dioxide. You can think of it as a buried natural boiler tank.

This combination of a permeable reservoir sealed by a solid caprock makes Campi Flegrei behave a bit like a pressure cooker. Heat from below turns water into superhot fluids. The sturdy caprock above keeps a lid on the system, so pressure can build up. When pressure rises enough, the ground above starts to lift (like a bulging lid on a pot about to boil over). Over the caldera’s long history, there have been times when that “lid” partially gave way: cracks opened, steam blasted out, and even small eruptions occurred. The last eruption here was in 1538 – a relatively small blast that created a new hill called Monte Nuovo. Historical accounts and geological evidence suggest the ground heaved upward several meters before that eruption, showing how extreme the “inflation” can get if the pressure cooker isn’t relieved in time.

In more recent memory, major uplift episodes have occurred without any eruption at all. In the early 1980s, Campi Flegrei’s floor rose by almost 2 meters, forcing thousands to evacuate from Pozzuoli, while swarms of earthquakes rattled the area. Yet no lava emerged; the caldera then slowly subsided over the next two decades. Since about 2005, the ground has been rising again—so much that by the mid-2020s it reached levels higher than those of the 1980s crisis. Clearly, something deep underground has been pumping up the surface like a tire being inflated, and then occasionally letting it leak or go flat.

Patterns in the Uplift and Quakes

Scientists have pored over data from these unrest episodes to see if there’s a consistent pattern. Lo and behold, the seemingly chaotic breath of Campi Flegrei does have a rhythm to it. By comparing the last two big unrest periods (the early 1980s and the 2010s–2020s), researchers identified a few key recurring features:

Uplift followed by subsidence: Each cycle begins with the ground lifting – sometimes slowly, sometimes in spurts – and eventually the trend reverses to sinking. It’s as if the caldera “inflates” and then partially “deflates” rather than just continually building toward an eruption.

Shallow quakes that get deeper over time: Earthquakes during these episodes tend to start at only 1–2 km below ground, mostly under the caprock lid. As time goes on and the unrest continues, new quakes start occurring a bit deeper, down around 3–4 km in the reservoir zone. The earthquake foci seem to migrate downward as the episode progresses.

A gassy reservoir around 3 km deep: Geophysical imaging (like seismic tomography) in both unrest periods has consistently spotted a zone between about 2 and 4 km down where seismic wave speeds are unusual (a low Vp/Vs ratio, in technical terms). That typically signals hot fluids or gas in the rock. Indeed, this corresponds to our buried boiler – a pocket of pressurized water and CO₂ at roughly 350°C. It’s a persistent feature, showing up in different studies years apart, so we know that a confined, gas-charged reservoir is a permanent player in this saga.

Silent depths below 5 km: Interestingly, little to no seismic action is seen between about 5 km deep and the magma zone around 8 km. In other words, during these unrest episodes, all the commotion stays in the upper few kilometers. If magma were actively intruding upwards, we’d expect to see earthquakes tracing a path from the deep magma zone up toward the surface. But we don’t – the deep zones stay quiet. This clues us in that magma itself isn’t directly squeezing upward each time the ground rises.

Strain builds up before quakes release it: The volcano’s crust seems to tolerate a lot of stretch before it snaps. In both recent uplift cycles, the local seismicity stayed relatively low while the ground was heaving upward – until the uplift reached a threshold (around 60–70 cm of rise). After that point, earthquakes picked up in frequency. It’s as if the rocks can elastically bend and accumulate strain energy up to a limit, then finally start fracturing and faulting once that limit is exceeded.

All these patterns paint a picture of a system where the primary action is in the shallow and middle crust, involving fluids and mechanical strain, rather than fresh magma on the move for each cycle. The challenge has been to understand what triggers and sustains this cycle. Why does the ground stop rising and start sinking? What causes the earthquakes to deepen? The answers lie in the interplay of water, rock, and heat.

Rainwater: Fuel for the Fire (or Rather, Steam)

Surprisingly, one of the key ingredients driving Campi Flegrei’s unrest may be plain old rain. Yes, the water falling from the sky – over months and years – percolates into the ground and joins the underground hydrothermal brewing. It might sound counterintuitive that rainfall can influence something as mighty as a caldera, but consider that water doesn’t just roll off into the ocean; a lot of it seeps down through soil and porous rock. In a caldera full of cracks and faults, rainwater can trickle its way deep underground, especially around the margins where limestone and other permeable rocks act like funnels.

Researchers analyzed 24 years of rainfall records around Campi Flegrei and noticed an upward trend in subsurface water recharge. Essentially, each passing year more rainwater is seeping underground than before. At first the porous rocks can absorb water like a sponge with little effect, but once they become saturated, any extra water has nowhere to go – so it increases the fluid pressure substantially.

In Campi Flegrei’s case, the rainwater (along with water released from deep magma as vapor and CO₂ gas) feeds into the reservoir below the caprock. Because the caprock is relatively impermeable when intact, the water can’t easily escape upward, so pressure builds. It’s analogous to pumping water into a sealed container – the more you pump in, the higher the pressure goes. This rising pore pressure in turn pushes the rock outward and upward (hence the uplift of the surface) and also makes it easier for cracks to form (since pressurized fluid can prise rock apart from within).

One might ask: doesn’t the water just boil off through fumaroles or find some escape? Some of it does vent out as steam in places like the Solfatara crater, yes. But the study suggests a lot of it remains trapped until a breaking point is reached. The trap is largely thanks to that sturdy caprock layer, and interestingly, the caprock gets even better at trapping over time because of a neat trick – it heals itself.

The Self-Healing Caprock – Nature’s Pressure Seal

Here’s a remarkable concept: Campi Flegrei’s caprock appears to self-seal cracks, almost like it’s bandaging its own wounds. This idea might sound a bit magical, but it has a real geochemical basis. The caprock is made of pyroclastic deposits (think volcanic ash and pumice from ancient eruptions) that have been chemically altered and cemented over thousands of years. It’s rich in minerals that can dissolve in hot water and then precipitate out.

When the pressure in the reservoir below builds enough, the caprock can crack – causing an earthquake or allowing some fluids to escape into overlying layers. But once the pressure is relieved a bit, hot mineral-rich water starts seeping into those new cracks. As it does so, it begins to cool or mix with other fluids, and minerals start coming out of solution, clogging up the fractures. Imagine a fracture in the rock becoming lined with new mineral growth, kind of like mineral “hair” growing to fill the gap. Over time, these fibrous crystals can completely seal the crack, restoring the caprock’s integrity.

In fact, scientists liken this to how Roman concrete was made. The Romans, who built many structures in the Campi Flegrei region, famously used volcanic ash (pozzolana) in their cement; when mixed with lime and water, it could set even underwater by growing durable mineral crystals. Now it seems nature is performing a similar trick underground: volcanic ash in the caprock, in the presence of hot, alkaline water and carbon dioxide, undergoes chemical reactions akin to cementation. The result is rapid healing – essentially the rock gluing itself back together. Tiny needle-like crystals (a bit like the fibers in fiberglass or the rebar in reinforced concrete) interlock within the pores and cracks, strengthening the rock again. This gives the caprock a fiber-reinforced structure that can hold pressure until the next cycle.

The new study provided evidence of this self-healing by examining rock samples and noting a fibrous microtexture in the Campi Flegrei caprock. But the researchers didn’t stop at inference – they went to the lab to reproduce it.

Brewing a Mini-Caldera in the Lab

How do you test a volcano’s plumbing in the laboratory? The researchers got creative. They built an apparatus mimicking Campi Flegrei’s setup, describing it as working “much like a moka pot.” (If you’re a coffee lover, you know that’s the classic Italian stovetop espresso maker.) In a moka pot, water in the bottom chamber heats up and is forced up through a funnel of coffee grounds, finally bubbling into the top chamber as coffee. In the experiment, the “coffee grounds” were actually crushed rock from Campi Flegrei – including volcanic ash and bits of lava – representing a fractured caprock. The bottom chamber contained a saline brine solution to simulate the mineral-rich geothermal fluid in the reservoir. The whole contraption was a strong pressure vessel, heated to around 200°C (comparable to the natural temperatures at 1–2 km depth in the caldera).

What happened? After about a day of letting this mini-volcano brew, the once loose, cracked rock in the funnel had cemented itself significantly. Hot pressurized fluid had risen through the crushed rock, and just like in the real caldera, it deposited minerals along the way. When the team took the assembly apart, they found that the previously fragmented ash and rock grains were now bonded by new mineral fibers and cements. Scanning electron microscope images revealed a mesh of fine fibrous crystals weaving through the pore spaces. In other words, the experiment demonstrated rapid self-healing of the rock under realistic caldera conditions – in just 24 hours, which is astonishingly fast. In nature, the process may occur over months or years, but the lab showed that given the right conditions, the chemistry is quick to act.

This lab verification is a big deal. It confirms that Campi Flegrei’s rocks have the capacity to reseal themselves on human timescales. Each time the volcano breathes out (releases pressure via fractures or small eruptions), it can essentially patch up its leaks, setting the stage for pressure to build again. Picture a leaky balloon that magically repairs each hole after it leaks air, and you get a sense of what’s happening under Pozzuoli.

A Fluid-Driven Cycle of Unrest

Combining all these pieces – the recurring uplift/quake patterns, the rainfall recharge, and the self-sealing caprock – we arrive at a coherent story of how Campi Flegrei’s bradyseism works. It goes something like this:

1. Recharge (Inhaling phase): During periods of heavy or increased rainfall (and continuous gas release from below), water percolates down and accumulates in the deep reservoir. The caprock, being mostly sealed, traps this fluid. Pressure begins to rise in the reservoir, like air being pumped into a tire. The ground above responds by lifting gradually. At first, the uplift can be mostly elastic (with few quakes) because the caprock holds together, flexing like a stretched spring.

2. Pressurization and Strain Buildup: As the reservoir keeps recharging, the fluid pressure inches toward a critical point. The rock’s strength is not infinite; eventually the stress of holding in that high-pressure fluid causes small fractures in the caprock or surrounding rock. Minor earthquakes start to pop off at shallow depths, signaling that the lid is starting to get leaky.

3. Release (Exhaling phase): Once enough cracks form or one larger fracture opens, it’s like a safety valve lifting on the pressure cooker. Hot water and steam rush into the cracks or upward. Some of it might even reach the surface as increased fumarole output or other hydrothermal activity. Critically, when pressurized water suddenly finds a pathway to shallower levels, it can flash into steam (since the pressure drops) – a bit like a pot boiling over. This rapid phase change can boost the pressure locally and provoke further seismic jolts. Earthquake focus shifts deeper for a time, because the stress is being transferred down to the edges of the reservoir as the caprock yields.

4. Subsidence and Relaxation: With the pressure partly relieved, the ground that was pushed up can settle back down – hence the subsidence observed after a peak uplift. The system has vented some steam, so to speak. If no new input of fluid happens, the area might even go quiet for years, slowly deflating as fluids drain or cool.

5. Self-Healing and Repeat: Here’s where the cycle resets. Those cracks that opened? Over the ensuing months to years, the hydrothermal system heals them by depositing minerals. The caprock regains its strength and tightness. Meanwhile, if the climate continues to add water or the deep system continues to bleed gases, the reservoir starts recharging again. The “inhalation” begins anew, and the whole process can repeat, perhaps over decades.

This model turns Campi Flegrei’s unrest into a repeating loop driven by fluid dynamics rather than one-off magma surges. It explains why the caldera can keep breathing without necessarily erupting each time: the pressure gets vented through cracks and small steam-driven explosions instead of one big magma eruption. It also explains the puzzling observation that after huge uplifts like in the 1980s, the ground can settle down again – something hard to square with magma intrusions (magma doesn’t just disappear, but water and gas pressure can dissipate).

Why These Findings Matter

Understanding this “pressure cooker” cycle isn’t just an academic exercise – it has practical implications for the millions living around Campi Flegrei and other calderas worldwide. Here are a few takeaways and why they’re important:

Improved hazard monitoring: Recognizing these patterns means scientists can better anticipate what’s next. For instance, if the ground has already swollen by roughly half a meter and heavy rains are ongoing, the system might be approaching that strain threshold when earthquakes usually ramp up. In short, keeping an eye on rainfall and groundwater could become as vital as tracking seismic and gas signals.

Phreatic eruption forecasting: The worst realistic scenario for Campi Flegrei short of a magma eruption is a phreatic explosion – basically a steam-driven blast that happens when superheated water suddenly flashes to steam and bursts out of the ground (think of a geyser-like explosion with no lava). These can be highly dangerous, as they often occur with little warning. By understanding that sealed pressure buildup is a precursor to such blasts, scientists can look for signs of sealing and pressurization (like changes in gas emissions or certain tremors) to better forecast a potential phreatic event. In other words, these findings help connect the dots between slow uplift and an eventual sudden steam explosion, potentially improving early warnings.

Rethinking magma’s role: This research shifts focus away from magma movements to the importance of hydrothermal fluids. Sometimes unrest is driven more by water and gas than by fresh magma. It’s a reminder that volcanoes aren’t just about fire and lava – they’re also about water and gas interacting with rock.

Engineering solutions to reduce risk: Perhaps the most outside-the-box idea raised is the possibility of intervening to alleviate the pressure buildup. If rainwater recharge is a key culprit, could we manage the water to manage the unrest? The authors suggest exploring ways to intercept or divert water flow before it percolates into the caldera’s depths. This might mean enhancing drainage or pumping groundwater in surrounding areas (for instance, in the Apennine foothills where a lot of rainwater sinks underground). Think of it like relieving a dam before it over-tops: by keeping some water out of the “pressure cooker,” we might reduce how often it tries to boil over. Any such plan would require careful study (and it wouldn’t be simple), but it’s a novel approach to hazard mitigation beyond the usual strategy of just evacuating and waiting.

General preparedness: Knowing that the ground rising isn’t a mysterious omen but part of a known cycle can help communities around Campi Flegrei prepare and respond calmly. It’s still unsettling that the ground moves, but at least it’s a step toward predictability. And if interventions (like water management) are feasible, local authorities and residents can be proactive in reducing risk.

Ultimately, Campi Flegrei’s “breathing” teaches us a broader lesson about volcanoes: they can operate on feedback loops with their environment. Here, the water cycle (rainfall) and geological processes are intertwined in a delicate dance. The volcano is not an isolated angry mountain; it’s part of an ecosystem, responding to and storing the rains that fall on it, only to spit that water back out as vapor later.

As we continue to study and monitor such systems, our ability to live safely in volcanically active regions improves. Campi Flegrei will undoubtedly continue its restless cycle – inhaling rainwater, building up pressure, and exhaling through quakes or steam – as it has for thousands of years. But armed with these insights, we can demystify its behavior. Who knew that the key to understanding a slumbering giant’s breath might lie in something as gentle as the rain, and in rocks that heal like living tissue? It’s a vivid reminder that our dynamic Earth often works in beautifully subtle and interconnected ways, where even a volcano’s heart can be influenced by the touch of a raindrop and the chemistry of ancient ash, stitching itself closed until the next deep breath.

Journal Reference:

1. Tiziana Vanorio, Davide Geremia, Grazia De Landro, Tianyang Guo. The recurrence of geophysical manifestations at the Campi Flegrei caldera. Science Advances, 2025; 11 (18) DOI: 10.1126/sciadv.adt2067