Bottini di Siena

The Bottini di Siena are a complex system of medieval underground aqueducts for the water supply of the city of Siena with a total length of 25 kilometres (16 mi). The system used to be the main water supply of the entire city of Siena until 1914 and nowadays continues to supply water to the fountains of Siena.

Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).

Other tunnels in Italy

Delaware Aqueduct United States · 137 km Päijänne Water Tunnel Finland · 120 km Orange–Fish River Tunnel South Africa · 82.8 km Ishikawa Tunnel Japan · 78.5 km Brenner Base Tunnel Austria · 64 km Mont d'Ambin base tunnel France · 57.5 km Gotthard Base Tunnel Switzerland · 57.09 km Hong Kong–Zhuhai–Macau Bridge China · 55 km