Built to bring water to the city of Lyon, the Roman aqueduct of the Gier was the longest of the four aqueducts present at the time. Built in the 1st and 2nd centuries AD, it ran for 86 kilometres and is nowadays one of the best-preserved of these structures. Excavations have made it possible to retrace its route, from Saint-Chamond in the Pilat massif to the Fourvière hill in Lyon. The aqueduct shows real ingenuity for its time, in particular thanks to the penstock technique which allowed it to cross steep valleys. The Beaunant siphon bridge is a fine example of this: it crossed the valley with a length of 270 metres and a height of 17 metres.
Chaponost in Rhône, more specifically the locality of Le Plat de l'Air, is where the most significant remains of the aqueduct are to be found: a long and majestic string of 72 arches running for over 550 metres is still visible there today.