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When we think of aqueducts, we usually picture rows of arches stretching for kilometres, once supplying the greatest cities of the Republic and later the Roman Empire with drinking water. Indeed, the arcades — many of which survive to this day — were part of that system; however, the majority of aqueducts ran underground: tunnels carved through rock, hidden from enemy eyes and generally far cheaper and more practical to build.
The Aqua Appia aqueduct, built in 312 BCE, was the first of many, measuring 16.56 km in length. Only about 89 metres of the original stretch of this water-supply system ran above ground; the rest ran beneath the surface, beginning on the slopes of the Alban Hills. The same was true of later constructions, whose above-ground sections were built mainly for prestige or because continuing underground was no longer feasible — in cities, this risked contaminating the water supply through local groundwater, while existing buildings and sewage channels would have made tunnelling difficult. But what were the sources of inspiration for such ambitious undertakings?
Etruscan neighbours
In building their aqueducts, the Romans — as in many other cases — developed ideas borrowed from neighbouring or conquered peoples. The same applied to water infrastructure, for which inspiration can be found, among other places, in Etruscan cuniculi: multi-purpose systems of water channels. In Veii, north of Rome, a settlement of Etruscan origin, approximately 22 kilometres of them have been discovered. Researchers Franco Ravelli and Paula J. Howarth, who studied the remains of cuniculi in Etruria and Latium, dated the finds to between the 8th and 3rd/4th centuries BCE, when the Etruscans still exerted considerable influence over the region.
The chronological overlap, cultural dominance, the proximity of Veii, and the employment of Etruscan builders all point fairly unambiguously to the fact that the Cloaca Maxima, built in Rome around 600 BCE, was a direct architectural borrowing. It was constructed under King Tarquinius Priscus, himself an Etruscan by birth, to drain excess water from Rome. Initially, following the model of Etruscan channels, it most likely had no roof; over time, however, as the city grew, it was built over and covered by a network of streets.
Cretan roots of hydraulics
Even before the Etruscans began building their channels, the Minoans — builders of the great palaces at Knossos — created the first complete water supply system in European history. It comprised two basic types: an open gravity system and a closed one made up of interconnected pressure pipes. The date of the earliest constructions is not known, but we can place them roughly between 1675 and 1450 BCE, the peak period of Minoan culture.
Minoan builders demonstrated a sound understanding of the principle of communicating vessels, applying it in aqueducts with a gradient estimated at 5%, which allowed water to be moved against the natural lie of the land. According to some researchers, the sanitary standards of Minoan cities were comparable to those of European metropolises in the second half of the 19th century CE.
The Minoan civilisation collapsed around 1100 BCE. Its hydro-engineering achievements, particularly gravity pipe systems, were largely forgotten for hundreds of years. Further development of similar concepts can be observed in the Near East, where they reached the scale of underground tunnels stretching tens of kilometres. These will be discussed in greater detail below.
Innovations from the east
Kanat, kārīz, and foggāra are just a few of the many names for water conduits widespread across the entire Middle East and North Africa. The oldest of them are believed to have been built on the territory of present-day Iran around the 10th–8th centuries BCE, making them contemporaries of the Etruscan constructions.
Qanats carried water from higher-lying mountain areas to large cities and agricultural fields in a climate far less favourable than that of the Apennine Peninsula. For this reason, a significant portion ran deep underground, thereby avoiding high temperatures and water evaporation. Connected to them were vertical access shafts that collected rainwater and water from numerous mountain springs and streams, while also allowing maintenance of the tunnels. Even in their final stretches, the channels and connected cisterns were often deep enough that water was drawn by means of wells.
An interesting additional application of qanats was the air-conditioning of residential spaces: hot air circulating through a building would mix in its lower sections with the cool draught from the qanat, reducing its temperature.
The jerwan aqueduct
The qanat technology mentioned above was further refined during the reign of the Assyrian king Sennacherib, under whom the Jerwan Aqueduct was built. Constructed between 703 and 690 BCE as part of a larger irrigation system supplying the gardens near Nineveh, this aqueduct was suspended over the Khenis river, connecting two sections of a traditional earthen canal. It is the oldest known structure of this type, and its scale and the techniques employed (stone arches, waterproof cement) demonstrate the precedence of Eastern concepts over their Mediterranean counterparts.
On the Mediterranean shore
Theagenes, tyrant of Megara, built in the 7th century BCE the first underground water-transport tunnel since the Minoan period. However, the true pinnacle of Greek hydro-engineering was the Tunnel of Eupalinos on Samos (6th century BCE). This 1,036-metre tunnel, bored simultaneously from both sides of a hill, remained in use until Byzantine times, and its interior can be visited today.
An important improvement introduced by the Greeks was the siphon, inspired by Pythagoras’s Cup of Justice. This vessel operated on the principle of communicating vessels. Inside was a vertical tube connected to a channel running through the stem. When the liquid level exceeded a certain height, a hydraulic siphon effect was triggered — the liquid flowed down on its own, emptying the entire cup. The same principle was applied in siphons used along sections of water channels that passed through mountain gorges, where a temporary drop was followed by a return to the original elevation of the conduit.
As with many other projects, Greek ambitions were constrained by technological limitations. Roman concrete, invented around the 2nd–3rd centuries BCE, marked a turning point for large-scale architectural projects: its binding properties allowed for more ambitious, slender, taller, and larger walls, arcades, columns, and even domes. It made possible the multi-storey arcades used by the Romans in constructing the above-ground sections of their aqueducts.
The techniques described above, though their evolution can be traced, were applied with varying frequency and in different configurations over the centuries. A Roman aqueduct in Syria more closely resembled a qanat than a contemporary aqueduct from Italy. Differences in climate and terrain played a key role in the construction of aqueducts; the sections with the least variation were the terminal stretches of the channels, which — for reasons of prestige and practicality — were raised above street level within urban areas.
