Rome could not feed itself. The city that consumed the products of an empire — grain from Egypt and North Africa, wine from Gaul and Spain, olive oil from the eastern Mediterranean, luxury goods from as far as India and China — sat on the Tiber sixteen kilometers from the sea, connected to the Mediterranean economy through a harbor at the river’s mouth and the logistical infrastructure that moved commodities from ships to warehouses to the city’s tables. Ostia was that infrastructure, and understanding it means understanding how an ancient city of half a million or more people solved the supply problem that has defeated urban civilizations throughout history.