What is "horreum"

A horreum (plural: horrea) was a type of public warehouse used during the ancient Roman period. Although the Latin term is often used to refer to granaries, Roman horrea were used to store many other types of consumables; the giant Horrea Galbae in Rome were used not only to store grain but also olive oil, wine, foodstuffs, clothing and even marble. By the end of the imperial period, the city of Rome had nearly 300 horrea to supply its demands. The biggest were enormous, even by modern standards; the Horrea Galbae contained 140 rooms on the ground floor alone, covering an area of some 225,000 square feet (21,000 m²). The amount of storage space available in the public horrea can be judged by the fact that when the emperor Septimius Severus died in 211 AD, he is said to have left the city's horrea stocked with enough food to supply Rome's million-strong population for seven years. Smaller (though similar) horrea were a standard feature of Roman towns, cities and forts throughout the empire; well-preserved examples of military horrea have been excavated on Hadrian's Wall in England, notably at the forts of Housesteads, Corbridge and South Shields.

The first horrea were built in Rome towards the end of the 2nd century BC, with the first known public horreum being constructed by the ill-fated tribune Gaius Gracchus in 123 BC. The word came to be applied any place designated for the preservation of goods; thus it was often used refer to cellars (horrea subterranea), but it could also be applied to a place where artworks were stored, or even to a library. Some public horrea functioned somewhat like banks, where valuables could be stored, but the most important class of horrea were those where foodstuffs such as grain and olive oil were stored and distributed by the state. Rome's insatiable demands for foodstuffs meant that the amount of goods that passed through some of the city's horrea was immense, even by modern standards. The artificial hill of Monte Testaccio in Rome, which stands behind the site of the Horrea Galbae, is estimated to contain the remains of at least 53 million olive oil amphorae in which some 6 billion litres (1.58 billion gallons) of oil were imported.