Crossing it hasn't been possible for centuries, but  Rome's Ponte Rotto – said to be the first stone bridge across the Tiber – still stands as a heroic fragment. Alden R. Gordon, professor of art history at Trinity College, Hartford, recently described the structure in an article published in the Wall Street Journal.

The bridge's original name was the Pons Aemilius (the Emilian Bridge), after one of the two men who built it, Aemilius Lepidus (Emilio Lepido), leader of the Roman Senate and Pontifex Maximus, the other being co-censor Fulvius Nobilior.

Previously, Rome's bridges had been made of wood and supported on timber piles driven deep into the riverbed. The new Pons Aemilius, by contrast, consisted of a wooden roadbed supported on five stone piers and embodied multiple feats of innovative engineering.

To build the piers, the Romans first had to master the creation of caissons, wooden forms built around pointed tree trunks driven into the riverbed. In the summer dry season and with the aid of pumps, the caisson could be emptied of water, permitting excavation through the riverbed to solid ground.

The bridge piers were made of opus quadratum, or ashlar masonry—squared blocks of stone set in parallel courses. They were bonded by being laid in an interlocking and overlapping pattern of identical slabs placed vertically or horizontally—so-called headers and stretchers. Metal clamps might also have been set into grooves to ensure the blocks would not shift laterally.

The stone used was locally quarried volcanic tufa, a compressed form of volcanic ash. It was easy to carve into the requisite blocks when damp out of the quarry, and achieved excellent compressive strength after drying in the sun for two years.

Read More and check out this video for a closer look.