Ancient Roman aqueducts are an engineering feat considered impossible at that time. Ancient Roman aqueducts were a unique engineering marvel at that time due to their history, construction, and impact on society. An aqueduct is an artificial bridge or viaduct that carries water. The aqueduct is derived from the Latin words "aqua" (water) and "ducere" (to lead) ("ancient Roman aqueducts"). Although aqueducts are a symbol of the technical progress of the Roman Empire, aqueducts are not a Roman invention. In 691 BC, the Persians invented a qanāt, which is a channel that carries water from deep underground to the surface. In 530 BC, the Greeks built a primitive aqueduct. Although the Romans did not invent aqueducts, they perfected them. At the height of the empire, eleven aqueducts supplied Rome: Aqua Appia, Aqua Anio Vestus, Aqua Marcia, Aqua Tepula, Aqua Julia, Aqua Virgo, Aqua Alestinia, Aqua Claudia, Aqua Anio Novus, Aqua Traiana and Aqua Alexandrina. These eleven aqueducts were built over a period of 538 years, from 312 BC to 226 AD (Schram, Wilke D.). The eleven aqueducts carried a total of 200 million gallons of water per day to Rome (Wulf, Carolina). The aqueducts provided each citizen of Rome with more than 265 gallons of water per day. This is far more than most modern water systems can offer today (Layton, Julia). In the early days of aqueducts, the water they brought was used in government buildings, mines, agriculture, and baths ("ancient Roman aqueducts"). As Rome's population grew, major aqueducts served government buildings, baths, public fountains, homes, and latrines throughout the city (Cote, Michael). The aqueducts of ancient Rome operated entirely on the forces of gravity. Every mile, the aqueduct descended a few feet to ensure a constant flow of water (Layton, Julia).