OSTIA, an ancient town and harbour of Latium, Italy, at the mouth of the river Tiber, on its left bank. It lies 14 m. S.W. from Rome by the Via Ostiensis, a road of very ancient origin followed by the modern road which preserves some traces of the old pavement and remains of several ancient bridges, until the construction of a new motor road to Ostia Mare. It was said to be the first colony ever founded by Rome—according to the Romans themselves, by Ancus Martius—and took its name from its position at the mouth (ostium) of the river. Excavations have, however, brought to light nothing earlier than the 4th cen tury B.C., the date of a small rectangular fort, measuring 200 by 125 yd., built of hewn blocks of volcanic stone, which may have been in Virgil's mind when he wrote his description of the forti fied camp which Aeneas founded at the Tiber mouth. It was out of this fort that the city developed the establishment of the salt-marshes (salinae—see SALARIA, VIA) which only ceased to exist in 1875. We learn much as to its cults, magistrates and trade guilds (for the last see J. P. Waltzing, Les Corporations professionelles [Brussels and Liege]) from the large number of inscriptions found. The city was divided into five regions. Vul can was the most important deity worshipped at Ostia, and the priesthood of Vulcan was held sometimes by Roman senators. The Dioscuri, too, as patrons of mariners, were held in honour. Later we find the worship of Isis and of Cybele, the latter being especially flourishing, with large corporations of dendrophori (priests who carried branches of trees in procession) and can ePhori (basket-carriers) ; the worship of Mithras, too, had a large number of followers. There was a temple of Serapis at Portus. At Portus a considerable number of Jewish inscriptions in Greek have come to light. In the 4th century Ostia began to be abandoned while the importance of Portus increased.
Until Trajan formed the port of Centumcellae (Civitavecchia) Ostia was the best harbour along the low sandy coast of central Italy between Monte Argentario and Monte Circeo. It is men tioned in the history of the year 354 B.C. as a trading port, and became important as a naval harbour during the Punic Wars.
Its commerce increased with the growth of Rome, and this, and the decay of agriculture in Italy, which obliged the capital to rely almost entirely on imported corn (which was, from 267 B.C. on wards, under the charge of a special quaestor stationed at Ostia), rendered the possession of Ostia the key to the situation on more than one occasion (87 B.C., A.D. 409 and 537). Ostia, however, was by no means an ideal harbour ; the mouth of the Tiber is exposed to the south-west wind, which often did damage in the harbour itself ; in A.D. 62 no less than 200 ships with their car goes were sunk, and there was an important guild of divers (urinatores) at Ostia. The difficulties of the harbour were in creased by the continued silting up, produced by the enormous amount of solid material brought down by the river. Even in Strabo's time the harbour of Ostia had become dangerous.
Caesar had projected remedial measures, but it was only under Claudius that the problem was approached. He constructed a large new harbour on the right bank, 2-1- m. N. of Ostia, with an area of 17o ac. enclosed by two curving moles, with an artificial island, supporting a lofty lighthouse, in the centre of the space between them. This was connected with the Tiber by an arti ficial channel, and by this work Claudius, according to his inscrip tions of A.D. 46, freed the city of Rome from the danger of inundation. The harbour was named by Nero Portus Augusti.
Trajan found himself obliged in A.D. 103, owing to the silting up of the Claudian harbour and the increase of trade, to con struct another port further inland—a hexagonal basin enclosing an area of 97 ac., with enormous warehouses and docks at tached, communicating with the harbour of Claudius and with the Tiber by means of the channel already constructed by Clau dius, which was prolonged so as also to give direct access to the sea. This became blocked in the middle ages, but was reopened by Paul V. in 1612, forming the right arm of the Tiber, by which navigation is carried on at the present day, and is known as the Fossa Trajana. The island between the two arms Procopius calls Insula Sacra (it is still named Isola Sacra).