The ancient Greeks and Romans knew this ground was dangerous. They called it the Phlegraean Fields -- the "burning fields" -- and the name was not metaphorical. The caldera is a nested structure composed of the older Campanian Ignimbrite caldera, the younger Neapolitan Yellow Tuff caldera, and scattered submarine and sub-aerial vents. It sits upon a fault system running northeast to southwest from the margin of the Apennine thrust belt, where the African and Eurasian tectonic plates converge. In 2008, scientists discovered that the Phlegraean Fields and Mount Vesuvius share a common magma chamber at a depth of 10 kilometers. The volcanic system that produced the Campanian Ignimbrite eruption is not extinct. Solfataras vent sulfurous gases, hot springs bubble, and the ground itself rises and falls in episodes of bradyseism -- slow, rhythmic deformation that periodically floods or exposes coastal ruins.

The eruption unfolded in a sequence that volcanologists have reconstructed from the deposits it left behind. It began with an intense Plinian phase in the northeastern sector of the caldera. The eruption column rose first to 29 kilometers, then peaked at 39 kilometers, before waning to 26 kilometers as the chamber began to exhaust itself. This Plinian phase lasted about 20 hours and tapped the uppermost, most chemically evolved trachytic magma. When the eruption rate exceeded the column's ability to sustain itself, the column collapsed, generating massive pyroclastic density currents that raced outward from the caldera. After the bulk of the magma had been expelled, the roof of the chamber gave way, and the caldera collapsed inward. This collapse triggered a final eruptive phase that drew from the deeper, less evolved portions of the magma chamber, depositing breccia and pumice along the caldera rim.

The eruption's consequences reached far beyond Campania. Petrological studies indicate the magma contained 50 to 250 million tonnes of sulfur dioxide, which the eruption injected into the stratosphere as sulfur aerosols. Climate simulations using the Community Earth System Model show temperature drops of 2 to 4 degrees Celsius across Western Europe in the year following the eruption, with peak cooling and acid deposition lasting one to two years. The eruption occurred near the onset of Heinrich Event 4, a millennial-scale cold period during which summer sea surface temperatures plummeted by 3 to 6 degrees along the Iberian margin and by 5 degrees in the westernmost Mediterranean. Some researchers have argued that the eruption's volcanic winter actually triggered Heinrich Event 4, compounding an already cooling climate into something far more severe.

For decades, researchers debated whether the Campanian Ignimbrite eruption contributed to the extinction of Neanderthals. The hypothesis was seductive: a supervolcanic event devastating enough to trigger a volcanic winter might have pushed an already declining species past the point of survival. But stratigraphic evidence complicates the story. At many European archaeological sites, the cultural transition from Mousterian tool traditions (associated with Neanderthals) to Uluzzian and Proto-Aurignacian traditions (associated with modern humans) begins below the ash layer left by the eruption -- meaning the transition was already underway when the caldera blew. In 2021, the timing of Neanderthal extinction was recalibrated to predate the eruption entirely. What the ash layer does mark, at site after site, is the abrupt end of transitional tool traditions. Above the tephra, the Early Aurignacian revolution begins -- a cultural shift that some researchers interpret as a direct consequence of the eruption's environmental disruption.

The Phlegraean Fields have erupted many times since the Campanian Ignimbrite event, most notably in the Neapolitan Yellow Tuff eruption around 15,000 years ago. The most recent eruption occurred in 1538, creating the cinder cone of Monte Nuovo. Today, nearly a million people live within the caldera's boundaries, and the system shows persistent signs of unrest. In 2016, Italian volcanologists announced plans to drill deep into the caldera, following up on the 2008 Campi Flegrei Deep Drilling Project that aimed to bore a 3.5-kilometer diagonal hole to collect rock samples and install seismic equipment. That earlier effort was suspended in 2010 due to safety concerns. The yellow tuff stone -- the ignimbrite itself, hardened into rock over millennia -- was quarried for centuries and used to build much of Naples, leaving the vast underground cavities that Neapolitans still use today. The city sits, quite literally, on the solidified remnants of its own potential destruction.