Over centuries, the mouth of the river shifted and the area became malarial, which led to it being abandoned and its buildings, particularly marble, plundered for other projects.
At its height it had 800 taverns and sailors were welcomed by throngs of prostitutes. Some of the city is yet to be excavated but even now it takes days to explore in detail.
Because few of the buildings have roofs it is hard to get a sense of how grand it must have been but a line of shops, each with a mosaic on the path outside to show its speciality (fishing equipment, elephants for construction work, grain imports), makes the busy commercial aspect of the city easy to appreciate.
If you don't have time to visit Pompeii or Herculaneum near Naples then Ostia is a great introduction to the way the ordinary Romans lived.
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Getting there: Cathay Pacific has daily flights from Auckland to Rome with a stopover in Hong Kong.
Further information: ostia-antica.org.