A restless supervolcano
The ground beneath the city of Pozzuoli, on the outskirts of Naples, is not stable. This area sits inside the Campi Flegrei, a 12-kilometer-wide volcanic caldera that has been "breathing" for millennia. This phenomenon, known as bradyseism, involves slow cycles of ground uplift (positive bradyseism) and subsidence (negative bradyseism). These movements are the surface expression of pressure changes in the vast magmatic and hydrothermal system that lies just a few kilometers below.
Two dramatic episodes of uplift occurred in the late 20th century. Between 1969 and 1972, the ground swelled by approximately 1.7 meters. A second crisis from 1982 to 1984 pushed the ground up another 1.8 meters, accompanied by over 16,000 earthquakes. This combined uplift of over three meters forced the evacuation of parts of Pozzuoli and caused significant damage to buildings and port infrastructure. After two decades of general subsidence, a new phase of slow uplift began in 2005.
Since 2005, the ground has risen by more than 1.2 meters. This ongoing uplift is accompanied by persistent seismic activity, with thousands of small earthquakes registered by the Vesuvius Observatory, which operates a dense monitoring network in the area. Scientists attribute this unrest to the intrusion of magmatic fluids into the shallow hydrothermal system, approximately 3 kilometers deep.
A natural geological laboratory
The evidence for Campi Flegrei's long history of bradyseism is etched into the local architecture. The most famous indicator is the Macellum of Pozzuoli, often called the Temple of Serapis. The three large marble columns of this ancient Roman market show a distinct band of holes drilled by marine mollusks (Lithophaga). These marks, found several meters up the columns, prove that the entire structure was once submerged due to ground subsidence before being uplifted again. This natural "seismic meter" was so compelling that geologist Charles Lyell used an illustration of the columns on the frontispiece of his foundational 1832 book, "Principles of Geology".
Modern monitoring by Italy's National Institute of Geophysics and Volcanology (INGV) is far more precise. A network of GPS stations, tiltmeters, and seismic sensors tracks ground deformation in real-time. Geochemists also analyze gas emissions from fumaroles, such as those at the Solfatara crater, measuring the composition of carbon dioxide and other volcanic gases to gauge the state of the magmatic system below.
The current activity does not signal an imminent eruption. The caldera remains under a yellow alert level. The last eruption within the caldera occurred in 1538, creating a new cinder cone, Monte Nuovo, in about a week. That event was preceded by decades of ground uplift. The ongoing surveillance aims to understand the complex processes at work and provide timely warnings for the half-million people living within the caldera.