In AD 373 the emperor Valens welcomed the waters of Thrace to the city of Constantinople, brought by a new aqueduct that still today bears his name. The water channel was over 150km in length and took nearly 30 years to build. Additions were made to the system over the next 100-150 years, bringing the total length of water channel provided for the city to somewhere in the region of 400km, representing one of the greatest achievements of hydraulic engineering known from antiquity. The longest stretch ran from the vicinity of modern Vize to Constantinople: at over 250 km in length this is the longest single water supply line known from the ancient world. More than 30 stone water bridges and many kilometers of underground tunnels carried the water over mountain and plain to the heart of the city. In many respects the completion of this new water-supply system inaugurated and confirmed the city as the new capital of the Roman world. Not only did it fulfill the daily needs of the growing population, but it also supplied the great thermae and nymphea, expected in any classical metropolis. Outside the city the archaeological reminders of this achievement survive in the forests of Thrace as impressive aqueducts and collapsed water channels. Within the walls over a hundred Byzantine cisterns have been identified, including three giant open-air reservoirs, attesting to the scale of the endeavour.