Pont du GardThe Pont du Gard at Nîmes in France is a bridge over a river carrying an aqueduct that supplied millions of gallons of water per day to the Roman city of Nîmes in Gaul; the water flowed in a channel at the very top. Although this bridge was built largely without mortar or concrete, many Roman aqueducts and bridges relied on concrete and sometimes iron rods for their strength. (Pont du Gard: © Masterfile Royalty Free)