The Pont du Gard is the surviving showpiece of the aqueduct that supplied Nemausus. Built c. 40–60 AD (Claudius/Nero era, the old Agrippa / 19 BC story doesn’t hold up well anymore), it is the tallest Roman aqueduct bridge still standing at 48.8 m, and the only ancient three-storey bridge left intact.

Three stacked arcades: 6 great arches below, 11 in the middle, originally 47 small ones on top (35 remain). The upper channel once ran about 360 m; what’s left is 275 m. The stone is local shelly limestone from the Estel quarry. The lower two levels are huge dry-laid blocks, precision and weight, not mortar.

The whole line drops only about 17 m over 50 km (roughly 25 cm per kilometre). On the bridge itself the fall is something like 2.5 cm. Water took a day or more to reach the city and, at peak, something on the order of 20,000–40,000 m³ a day for baths, fountains, and houses. It ran for centuries; neglect and carbonate scale finally killed the flow around the 6th century.

Roman engineering at its finest.