Ostia Antica, the Port of Rome

Built sometime in the 4th century BCE, Ostia was the primary port for both commercial and naval shipping for the Republic and Empire, until Portus was constructed in the 1st century CE. A complete town, with it's own forums, temples and theater, it was also full of warehouses for storing goods transitioning between land and sea transport. After Portus became the primary port faciluty, Ostia transitioned into more of a resort town and transhipper of luxury items. The shifting course of the Tiber both flooded the cit repeatedly and moved the seafront and river shore away from Ostia. By the 9th century CE, the town was abanodoned. Repeated flooding covered everything with layers of preserving silt. The town was excavated in the late 1930s.

There is a Virtual Tour here, and good info here, as well as a cool 3D reconstruction.