Ostia ŏs´tēə [key], ancient city of Italy, originally at the mouth of the Tiber but now inland as the Tiber delta has grown. It was founded (4th cent. BC) as a protection for Rome, then developed (from the 1st cent. BC) as a Roman port, rivaling Puteoli. Augustus, Claudius I, Trajan, and Hadrian expanded the city and harbor. From the 3d cent. AD the city began to decline. The ruins, of great archaeological interest, rival those of Pompeii in showing the layout of an ancient Italian city; significant excavations began only in the early 20th cent.
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