The Roman aqueduct system is the most visible surviving example of ancient hydraulic engineering, but the underground portions — the actual channels carrying water through mountains and across valleys — are often more impressive than the famous arcaded structures visible above ground.

The Aqua Claudia, begun under Caligula and completed under Claudius, carried water from springs in the hills east of Rome nearly 70 km to the city. Of this distance, only about 15 km were carried on arched bridges; the rest was underground channel. The engineering challenge of maintaining a consistent gradient — enough fall to keep water flowing but not so much that it damaged the channel — over 70 km of varied terrain was accomplished with surveying technology that remains impressive.

Frontinus, appointed curator of the aqueducts under Nerva in 97 AD, wrote a technical treatise on the Roman water supply that is both a historical document and an engineering manual. His inventory of the Roman system — nine aqueducts delivering hundreds of thousands of cubic meters per day — gives us the best documented picture of an ancient urban water supply.

The distribution of water within Roman cities is less well documented but equally interesting. Lead pipes (fistulae) of varying diameters distributed water from receiving tanks (castella) to public fountains, baths, and wealthy private households. The presence of lead in Roman drinking water has been proposed as a cause of lead poisoning in Roman society, though the actual exposure levels are disputed.