The stone arches looped solemnly over their shadows, some teetering above the grass, some sinking into it. It was a dazzling January morning, and I was standing in the Park of the Aqueducts, about 20 minutes by metro from central Rome.

Here, the ruined arcades of six of the 11 aqueducts that once supplied the Eternal City with an astonishing volume of water — by some counts double the per capita water allotment of a typical 21st-century American city — have been preserved.