crater lake in the Alban Hills (Colli Albani), southeast of Rome. Elliptical in shape, formed by the fusion of two ancient volcanic craters, it lies 961 feet (293 m) above sea level and has an area of 2 square miles (5 square km) and a maximum depth of 558 feet (170 m). It is fed by underground sources and drained by an artificial outlet, reputedly built in 398–397 BC because the oracle at Delphi said that the Etruscan stronghold of Veii could be taken only when the waters of the lake reached the sea. Among the lakeside towns, the best known are the resorts of Albano Laziale and Castel Gandolfo, the papal summer residence.