The Phlegraean Fields is a large volcanic caldera west of Naples, Italy. The Neapolitan Yellow Tuff eruption produced just 50 cubic kilometers. It is, however, one of relatively few volcanoes large enough to form a caldera. It is part of the Campanian volcanic arc, which includes Mount Vesuvius, about 9 km east of Naples. The Phlegraean Fields is monitored by the Vesuvius Observatory. Part of the city of Naples is built over it.
The Phlegraean Fields' largest known eruptions have an estimated volcanic explosivity index of 7. From Wikipedia
