The Phlegraean Fields is a large caldera volcano 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. It was declared a regional park in 2003.