Campi Flegrei (in English, the “Phlegraean Fields”) is a massive caldera, or collapsed volcano, encompassing much of the Gulf of Naples and surrounding coastal area. Calderas are a more insidious type of volcano than “stratovolcanoes” like Vesuvius because they don’t have one, obvious vent or a central peak; instead, a huge magma chamber deep underground feeds features such as cinder cones, craters, and fumaroles across a large area. To make matters worse, when calderas really blow, these are among the most explosive and destructive volcanic eruptions.

Campi Flegrei has had two major eruptions: one approximately 15,000 years ago, known as the Neapolitan Yellow Tuff, and one about 39,000 years ago, called the Campanian Ignimbrite. Both were global events, not only devastating the region but causing climatic changes; temperature drops due to the Campagnian Ignimbrite may have contributed to the extinction of the Neanderthals. Campi Flegrei’s last eruption, in 1538, was a relatively minor one – though it was forceful enough to form a new mountain, the aptly named Monte Nuovo.

In the intervening half-millennium, the volcano has been far from dormant. At INGV’s Vesuvius Observatory, just a few minutes away from the CFDDP’s pilot borehole, staff monitor the activity of Campi Flegrei (and Vesuvius) around the clock. One wall is covered with screens showing shudders of seismic activity around the region; the digital graphs crawl across the monitors like EKG lines. On another side of the room, a satellite map of Campi Flegrei displays the seismic events of the last two years; dozens of dots representing tremors are clustered over the most active area of the caldera. The volcano is ever reminding Neapolitans of its presence.