OF ALL the feats of ancient engineering, Roman aqueducts are among the most remarkable. “With such an array of indispensable structures carrying so many waters, compare, if you will, the idle Pyramids or the useless, though famous, works of the Greeks!” wrote Sextus Julius Frontinus (35–c. 103 C.E.), Roman governor and water commissioner. *

Ancient cities were usually built near an abundant water supply, and Rome was no exception. Originally, the Tiber River and nearby springs and wells provided sufficient water. From the fourth century B.C.E. on, however, Rome grew rapidly, as did its need for water.

Since few people had running water in their homes, the Romans built hundreds of private and public baths. The first public bath in the city of Rome was fed by the Aqua Virgo, dedicated in 19 B.C.E. The builder of this aqueduct, Marcus Agrippa, a close friend of Caesar Augustus, poured much of his vast fortune into overhauling and extending Rome’s water-supply system.

Baths also became social venues, larger ones even having gardens and libraries. After leaving the baths, aqueduct water, which could not be shut off, flowed into the sewers, constantly flushing them of refuse, including waste from the latrines attached to the baths.

When you hear the words “Roman aqueduct,” do you think of lofty arches running to distant horizons? In fact, arches formed less than 20 percent of those conduits, the larger portion of which lay underground. This more economical design not only protected aqueducts against erosion but also minimized their impact on fields and neighborhoods. For example, the Aqua Marcia, completed  in 140 B.C.E., was about 57 miles (92 km) long but comprised just 7 miles or so (11 km) of arches.

Before building an aqueduct, engineers assessed the quality of a potential water source by examining the clarity, rate of flow, and taste of the water. They also took note of the physical condition of the locals who drank it. Once a site was approved, surveyors calculated the right path and gradient for the conduit, as well as its channel size and length. Slaves evidently provided manpower. Aqueducts could take years to complete, making them costly—especially if arches were needed.

Moreover, aqueducts had to be maintained and protected. To care for them, the city of Rome at one time employed about 700 people. Provisions for maintenance were also incorporated into the design. For instance, underground sections of the aqueducts were made accessible by means of manholes and shafts. When major repairs were needed, engineers could temporarily divert the water away from a damaged section.

By the early third century C.E., 11 major aqueducts served the city of Rome. The first, the Aqua Appia, built in 312 B.C.E. and just over ten miles (16 km) long, ran almost entirely underground. Still preserved in part is the Aqua Claudia, which was some 43 miles (69 km) long with about 6 miles (10 km) of arches, a number of which stood 90 feet (27 m) high!

How much water did the city’s aqueducts carry? A lot! The Aqua Marcia, mentioned earlier, daily channeled about 6.7 million cubic feet (190,000 cu m) of water into Rome. Once the water reached urban areas—gravity being the driving force—it flowed into distribution tanks and then into branches, which channeled the water to other distribution tanks or to locations for water use. Some estimate that Rome’s water distribution system grew to the point that it could have daily supplied more than 265 gallons (1,000 L) of water for each inhabitant.

As the Roman Empire grew, “the aqueducts went wherever Rome went,” says the book Roman Aqueducts & Water Supply. Travelers in Asia Minor, France, Spain, and North Africa can still gaze in awe at these ancient marvels of engineering.