When Constantine the Great refounded the Greek city of Byzantium in 330 as Constantinople, the new imperial capital had a problem its old Greek seed had never needed to solve: it was vastly bigger than the available water. The first emperor to start fixing this had been Hadrian, two centuries earlier, but Constantine's expansion outran what existed. The full system the late Roman emperors built around the Aqueduct of Valens eventually stretched more than 250 kilometers through the hill country of Thrace, the longest aqueduct system of antiquity. The first phase, finished in 373, drew water from springs at Danımandere and Pınarca and ran 268 kilometers to the city. A fifth-century second phase added another 451 kilometers of conduits reaching all the way to Vize, 120 kilometers out. Once inside Constantinople, the water filled three open reservoirs and over a hundred underground cisterns - including, eventually, the famous Basilica Cistern with its forest of recycled columns. Combined capacity ran into the millions of cubic meters.

There is a darker thread in the story of how the bridge itself was built. According to a tradition recorded by ancient writers, the stones came from the walls of Chalcedon, the city directly across the Bosphorus from Constantinople. Chalcedon had backed the wrong man - a usurper named Procopius, who briefly contested Valens for the throne. After the revolt collapsed in 366, Valens ordered the city's walls torn down as collective punishment, and the dressed stones, instead of being abandoned, were ferried across the strait and recycled into the imperial water system rising in Constantinople. Whether literally true in every block or partly mythic, the story captures something real about late Roman power: walls of one city pulled apart to lift a bridge in another, the stone itself made an instrument of imperial will. The mason who set those blocks in 373 might have recognized them. Some of them are probably still in place above the boulevard today.

The aqueduct's working life was extraordinarily long. The Avars cut it during their siege of Constantinople in 626 - a calculated act of pressure on a thirsty city - and water did not flow again until Emperor Constantine V repaired the system after the great drought of 758. By 1075, it was being administered by an official named Basil Maleses, who had been captured by the Seljuk Turks at the catastrophic Battle of Manzikert four years earlier and somehow returned to Constantinople to take up his duties as keeper of its water. Crusaders passing through in 1096 saw it running. So did Odo of Deuil during the Second Crusade. After 1453, the Ottomans inherited the aqueduct along with the city, kept it in service, called it Bozdoğan Kemeri, and continued to draw water through its arches. As late as the 1950s, modern surveys recorded daily discharge of more than six thousand cubic meters - well over a millennium and a half after the legions of Valens cut the ribbon.

Today the aqueduct does the unspoken work of any beloved old monument in a working city: it is a landmark, a backdrop, a point of orientation. Drivers on Atatürk Boulevard pass beneath the arches without thinking about Procopius or the walls of Chalcedon. Tourists pause for photographs. Cats sleep on the lower courses where the brick has weathered to a soft russet. The 18 to 73 numbered arches have a double order; the others a single order. The masonry is irregular - ashlar blocks mixed with brick, the work of generations of repair across many empires - and that irregularity is part of why it has survived. A Roman aqueduct made into a Byzantine aqueduct made into an Ottoman aqueduct, kept alive because each new occupant of the city needed water, and the simplest way to keep getting water was to keep the bridge in working order. Sometimes the great achievements of antiquity are great precisely because we never stopped using them.