volcanic crater, Napoli provincia, Campania regione, southern Italy. It is situated in the Campi Flegrei volcanic region just west of Naples. The crater, about 4 miles (6 km) in circumference, was known to the Greeks and Romans for its hot springs and the six-story Thermae Anianae (“Thermal Baths”) built for visitors. However, in the Middle Ages a lake formed in the crater, and this was not drained until 1866. Water draining from the crater still creates numerous mineral springs of various temperatures that are utilized by a large spa at Agnano Terme on the crater's south rim.