[23, Park of the Aqueducts, Rome] As Rome expanded, they built elaborate waterworks — aqueducts you can see to this day — bringing fresh water into the great cities of the empire: to Nîmes in France, to Segovia in Spain, and of course into the city of Rome itself.

[86, Pont du Gard aqueduct, mid-first century AD, France] Water infrastructure was a Roman engineering forte…and vital for the empire. The Pont du Gard (in southern France) is just one of many ancient aqueducts surviving across Europe. They heralded the greatness of Rome, reminding the far-flung empire's subjects how fortunate they were to be on the winning team. This perfectly preserved Roman bridge supported a canal, or aqueduct, on the very top. The Pont du Gard was a critical link, helping keep a steady river of water flowing cross-country and across this river. Remarkably, it was engineered so that the water dropped only one inch for every 300 feet for 30 miles, harnessing gravity to flow all the way to the city of Nîmes. A chance to walk through the top level shows how it all worked.

[87, inside Roman Aqueduct, Pont du Gard, France] This is what Roman aqueducts were all about. This is part of a 30-mile long channel — a man-made river flowed through this for 400 years. You can still see the original stones, a thin layer of mortar that waterproofed the channel, and, after centuries of use, a thick mineral build-up.

[88] The Pont du Gard's main arch is the largest the Romans ever built. The bridge itself has no mortar, just ingeniously stacked stones. Taking full advantage of that Roman specialty — the round arch — the structure is held in place by gravity.

[89] Simple as it may seem, the round arch was key to Roman architectural greatness. Previous structures were limited by two vertical posts spanned by a lintel, which was structurally weak. A round arch could span a much wider gap. And once the central keystone is placed, the arch can support just about whatever you want to build on top of it. Without the round arch, none of Rome's greatest structures would have been possible.