Campi Flegrei, also known as the Phlegraean Fields, is a large 13 km (8 mi) wide caldera. Today most of the area lies underwater, but it includes the town of Pozzuoli and the Solfatara crater, mythological home of the Roman god of fire, Vulcan. It is thought that the caldera was created in two major events. The first occurred about 40,000 years ago, erupting about 200 km³ of magma (500 km³ bulk volume) to produce the Campanian Ignimbrite. Approximately 12,000 years ago another major eruption occurred, forming a smaller caldera inside the main one, centered on the town of Pozzuoli.
Patrick Moore used to cite Campi Flegrei as an example of why the impact craters on the moon must be of volcanic origin, which was thought to be the case until the 1960s.