Phlegraean Fields: A hidden cavity discovered beneath the caldera.

UA previously unknown cavity, nestled 3,6 kilometers deep beneath the Phlegraean Fields, could offer new answers on the dynamics of the Neapolitan volcanic system and on the management of seismic and volcanic risk.

It was identified by an international team led by the University of Pisa, in collaboration with the National Institute of Geophysics and Volcanology (INGV) and the German research center GFZ – Helmholtz Centre for Geosciences in Potsdam. The results of the study were published in the scientific journal Nature Communications Earth and Environment.

The discovery concerns a fracture approximately one kilometer long, 650 meters wide and with an average thickness of 35 centimeters, for a total volume of approximately 220.000 cubic meters. A structure that, according to the researchers, connects the deep magma reservoir with the superficial fumaroles of Solfatara and Pisciarelli.

The exact contents of the cavity are not yet known, but the presence of high-pressure gases or magmatic fluids is hypothesized.

“We identified this structure by analyzing very long-period seismic signals, which have consistently resonating at the same frequency (0,114 Hz) since 2018,” explains Giacomo Rapagnani, a PhD student at the University of Pisa and the study's lead author. “Its consistency over time indicates that its size and composition have remained stable, a crucial piece of information for monitoring any structural changes that could signal an increase in volcanic risk.”

The Campi Flegrei area, one of the most closely monitored in the world, has been experiencing a phase of ground uplift—known as bradyseism—since 2005, accompanied by a growing seismic swarm. The strongest earthquake recorded so far occurred on June 30, 2025, with a magnitude of 4.6.

The study analyzed over one hundred earthquakes from the past seven years. "It emerged that during the most intense events, a low-frequency resonance is activated," Rapagnani adds, "which revealed the presence of the fracture. Similar behavior has been observed at other active volcanoes, but has never before been documented at Campi Flegrei."

According to Francesco Grigoli, co-author of the study and professor of Geophysics at the University of Pisa, "this research demonstrates the importance of developing and applying advanced techniques to analyze large amounts of seismic data. Only in this way can we better understand complex geophysical processes and increase our ability to prevent disasters."

The study represents a step forward in understanding the activity of the Phlegraean supervolcano and offers new tools for risk assessment in a densely populated and highly vulnerable area. The study's authors also include Simone Cesca, Gilberto Saccorotti, Gesa Petersen, Torsten Dahm, Francesca Bianco, and Francesco Grigoli.