Actually, we're not just talking about millions, but about hundreds of millions of big pots. They represent perhaps 6.5 billion liters of olive oil imported to Rome in the first 250 or so years AD. The pile is 50 meters deep (30 meters above the current ground level) and 1500 meters in circumference. That's about 22,000 cubic meters of broken pottery. Almost all of them are the same kind of amphora with an almost spherical body and very different from the longer thinner wine and grain amphorae you see around archeological sites and museums. The pottery is also noticeably thicker and coarser -- they were designed to be strong and to be used only once. Almost all of them come from the same area that the oil came from, ancient Baetica, now Baezia, in southern Spain. In those days, Spain was the largest producer of olive oil in the world.
The hill of pottery is accessible for those of you who want to stand at the top of hundreds of millions of pots. There are, needless to say, potsherds everywhere. Deeper into the pile, where archeologists dig, there are complete pots and many large fragments. Almost every pot was incised or stamped with a maker's mark, and the various southern Spanish pottery factories are thoroughly documented, so, if you find a piece with writing on it, it's probably neither unique nor particularly valuable.
So what did the ancient Romans do with all that oil? Like modern Mediterranean peoples (and other folks worldwide who were slower on the uptake) they ate a lot of it. Edible olive oils (low acid content) were processed much as they are done today. "Cold pressing" technology was the rule for higher quality oils, but they also knew you could get "second pressing" oils by adding hot or boiling water to the mats of skins, seeds, and pulp and pressing them again. The quality of the oil was not particularly well regulated, but market mechanisms apparently were quite good -- people quickly found out who was selling preferred oils and those dealers prospered. Almost all modern olive oils are pressed (or centrifuged) from ripe olives. Ancient Romans had five different categories based on the maturity of the fruit ranging from albis (from green olives) through virde (starting to ripen), maturum (fully ripe), and caducum (fallen) to cibarium (rotten) the last being fed mostly to slaves and ignorant foreigners.
Inedible olive oils lit Rome. Oil lamps were ubiquitous in public and private buildings and on the streets -- in some periods of the empire, building owners were required to provide the latter. They weren't very bright, but many could be lit if you were rich. Often lamps were used with mirrors and reflectors to concentrate light.
Another use for inedible oils (and for high priced edible oils if you were rich), often mixed with scent, was in the bath. Especially in public baths and gymnasia, oils were rubbed in and then scraped off the body with a curved strigilis to remove soil and dust. Oils were often used instead of soaps, and they actually worked quite well.
Perhaps the most important use of oil was that it made vast fortunes for Roman and Spanish families that held shares of the cartel. Emperors consistently got a cut of the profits, and members of oil cartel families consistently held the governorships in Spain. Similar deals were in force for Egyptian grain, British tin and silver, German silver, Middle Eastern spices, scents and incense, and just about all other commodities used by the empire. They were cozy economic deals that for centuries contributed to the unity of the empire.
The Mount Testaccio pottery has been very well studied, and, not surprisingly, much of the study has been financed by the modern Spanish olive oil industry and carried out by Spanish University Archeology Departments. The best Mt. Testaccio web site is from the University of Barcelona and the English version is at: http://www.ub.es/CEIPAC/MOSTRA/u_p01.htm (it continues on 13 more linked pages.)
The amphorae -- almost all "Dressel 20": http://intarch.ac.uk/journal/issue1/tyers/DR20.html and http://www.abc.se/~pa/mar/dressel.htm
Everything worth knowing about olive oil: http://www.oliveoilsource.com/olive_facts.htm (production statistics and lots of olive oil links.)
Oils and soaps: http://members.tripod.com/~marieainsley/instruction/soap.htm
P.S.: 1. Myth: Romans scrubbed down with oil because they didn't know about soap. Fact: The earliest known Mediterranean soap recipe is more than 2500 years old. Despite this, the Romans claimed they had invented it. They didn't, but they did have a plausible "discovery" myth: storms were said to have washed mixtures of animal fats and ashes down from the sacrificial temples atop Mount Sapo into the Tiber River in the ancient Roman past. Folks bathing there noticed they got cleaner, and an industry was born. The myth is certainly where the English word soap came from -- through Latin/Italian and probably French. The ancient Roman historian Pliny wrote about soap-making and gave a reasonable recipe featuring goat tallow, causticized wood ash, and common salt as a hardener. A full-scale soap factory, including finished bars, has been excavated at Pompeii. Use of soaps in public baths was probably frowned on because, when mixed with Italy's hard water, they made a scummy flaky precipitate that would contaminate the pools -- we've all seen it here in our own tubs.
2. Mt. Testaccio was the scene of Carnival, Lenten observances, and Easter ceremonies in Medieval times. A winding path up the hill led to Christian crosses, and the hill was known as the Roman Calvary. In Renaissance times these observances were transferred to other Roman locations, and they were finally abandoned completely when Rome became a secular city after 1870.
3. Spain apparently always has been the world's biggest olive oil producer, and today its 190 million trees produce about 550 million liters of oil per year, thirty percent of the world's production. Italy is the next largest producer with 24 percent of world production. Italy exports premium oils but, nonetheless, is a net importer of olive oil using millions of liters more than it produces every year. The US is way down the list and produces one tenth of one percent of the world supply. US agricultural technology, however -- particularly the use of hormones to regularize ripening times -- has vastly increased world-wide production. The world uses 1.94 billion liters of olive oil per year but only produces 1.84 billion. We have to make more, and fast, or we'll run out.
4. Those round oil jugs that make up Mt. Testaccio each held 60-65 liters of oil. The globular shape ensured maximum strength and made for easy stowage aboard the specially constructed ships that carried them -- neck size was dictated by the size of the spaces between the curves of the jugs in the next higher layer. The same cartel that controlled Spanish oil imports for 250 years also controlled the pottery factories so standardization of pots and ships was easy.