A circular lake, 5 m. W. of Naples, Italy. It was apparently not formed until the middle ages, as it is not mentioned by ancient writers; it was drained in 1870. It occupied the crater of an extinct volcano, 4 m. in circumference. On the south bank are the Stufe di S. Germano, natural sulphureous vapour baths, and close by is the Grotta del Cane, from the floor of which warm carbonic acid gas constantly rises to a height of 18 in., the fumes of which render a dog insensible in a few seconds. It is mentioned by Pliny (Nat. Hist. ii. 93). Remains of an extensive Roman building and some statues have been discovered close by.