The Campi Flegrei is dangerous. I'm not saying this to scare anyone or fear monger, but any time you have a large, restless caldera volcano with over 3,000,000 people living within ~19 miles (30 kilometers), you should be nervous. It would kind of be like moving Yellowstone to the outskirts of Tampa, Denver or San Diego. You'd want to know as much as you can about what is going on deep below your feet ... and what signs means an eruption might be coming soon.
That is exactly what we have near Naples in Italy, except that we can also add Vesuvius into the mix (so plot Mt. Rainier next to Tampa as well). The Campi Flegrei and Vesuvius are not linked directly, but they are both products of the complicated tectonics of Italy. To distill it down, the African Plate is subjecting (sliding under) the European Plate, creating small amounts of magma under the base of the crust. This melting feeds the volcanoes of Italy, including Campi Flegrei, Vesuvius, Etna, Stromboli and more.
The Notorious Caldera
The Campi Flegrei has produced some of the largest explosive eruptions Europe has seen in the past 50,000 years: the ~36,000 year old Campanian Ignimbrite that dumped almost 70 cubic kilometers of volcanic debris across the continent and the ~15,000 year old Neapolitan Yellow Tuff that added another 40 cubic kilometers on top.
