The huge numbers of broken amphorae at Monte Testaccio illustrate the enormous demand for oil of imperial Rome, which was at the time the world's largest city with a population of at least one million people. It has been estimated that the hill contains the remains of as many as 53 million olive oil amphorae, in which some 6 billion litres (1.3 billion imperial gallons/1.6 billion U.S. gallons) of oil were imported. Studies of the hill's composition suggest that Rome's imports of olive oil reached a peak towards the end of the 2nd century AD, when as many as 130,000 amphorae were being deposited on the site each year. The vast majority of those vessels had a capacity of some ; from this it has been estimated that Rome was importing at least 7.5 million liters (1.6 million imperial gal/2 million U.S. gal) of olive oil annually. As the vessels found at Monte Testaccio appear to represent mainly state-sponsored olive oil imports, it is very likely that considerable additional quantities of olive oil were imported privately.