(Baia). A city in Campania; situated on an inlet toward the western end of the Gulf of Cumae (Bay of Naples), and separated by a neck of land from Cumae (Cuma, on the Tyrrhenian Sea), of which it may originally have been a dependency. It owed its name, according to legend, to Baios the navigator of Odysseus.

Situated on a hillside sloping down to the shore, the place became a highly fashionable resort for wealthy Romans in the 1st century onward, because of tis medicinal springs (already exploited from c 200 or earlier), and mild climate and attractive surroundings. Owners of houses in the neighborhood included Marius, Cicero, Pompey the Great, Julius Caesar, and the emperors Gaius (Caligula), Nero and Severus Alexander. Gaius (AD 37-41) had a famous bridge of boats constructed from Baiae to Puteoli (Pozzuoli) for an extravagant parade. Nero (54-68), whose palace possessed its own oyster beds and fishing-lakes (depicted on glass bottles), used the pretext of the annual festival of Minerva at Baiae to invite his mother to dinner there, before arranging for her murder at Bauli nearby (59); and he allegedly poisoned his aged aunt Domitia in order to inherit her Baiae estates, on which he constructed a handsome gymnasium. Hadrian died in the neighborhood in 138. A large part of the region was imperial property.

Cicero identified Baiae with every sort of luxurious entertainment and loose living. Horace praised its alluring charms; and descriptions of its villas are provided by Seneca and Martial. Excavations have revealed a vast group of buildings, extending over two centuries, which have been identified as portions of a palace constructed by one of the emperors; three rooms, the 'Stanze di Venere,' are notable for their stuccoed vaults. Adjoining monumental complexes have been interpreted as separate but coordinated thermal establishments: the istitutions which made Baiae, in the late Republic and early Principate, the most important and famous medicinal bathing resort in the Mediterranean world, equipped with every curative, residential and recreational amenity. These buildings have, in modern times, been named the Baths of Sosandra, Venus and Mercury. The so-called Baths of Sosandra (or Aque della Rogna) are on three levels, united by grand staircases and ramps. A 'Temple of Venus' is externally square but internally circular, and a large 'Temple of Mercury' has an octagaonal exterior. Like other parts of the so-called Baths, these three 'temples,' or the first two of them, have generally been identified as portions of the thermal establishments mentioned above, but it has lately been suggested that they instead formed part of the imperial palace. Recent finds include a well-preserved statue of Dionysus, discovered under water, and an unusual assemblage of plaster statuary, comprising cats of Greek bronze originals, intended for copyists at work on marble reproductions.

Baiae, like neighboring Puteoli, experienced (as it still does) the gradual earth movements knwon as bradyseism (slow earthquake), and since Roman times the sea has moved more than a hundred yards inland. These encroachments and other seismic disturbances, together with the fevers which (according to Cicero) had already been present in Republican times, eventually caused Baiae's decline and ruin, although the use of its therapeutic waters continued without intermission until modern times.