The Romans built bridges the way they built everything: systematically, durably, and in sufficient quantity that their combined effect transformed the physical landscape of three continents. More than 900 Roman bridges have been identified by archaeologists, ranging from small rural crossings to major river spans, and approximately 700 of these survive in some form. The number is less remarkable than the durability: bridges that have been carrying traffic — first Roman, then medieval, then modern — for two thousand years represent an engineering achievement that no subsequent civilization has equaled in pure longevity. Several Roman bridges in active use today are the oldest functioning bridges in the world.

Frontinus, the Roman senator appointed curator aquarum — superintendent of waters — in 97 AD, opened his report on Rome’s water supply with a sentence that has been quoted many times since: compare, if you will, the idle pyramids, or the useless though famous works of the Greeks, with these many indispensable structures. The arrogance is characteristic, and the comparison is not entirely fair. But the underlying point is not wrong. Rome’s aqueduct system was among the most impressive engineering achievements of the ancient world, and it was, unlike the pyramids, entirely functional — designed to do something specific, doing it at scale, and doing it for centuries.

The Cloaca Maxima — the Great Drain — is among the oldest continuously functioning pieces of Roman infrastructure. Built initially in the sixth century BC to drain the marshy valley between the Capitoline and Palatine hills that would become the Roman Forum, it has been carrying water to the Tiber for over 2,600 years. Tourists floating on the Tiber can still see its outlet — a rounded arch of tufa stone nearly four meters high, set into the river embankment near the Forum Boarium — and the drain itself, though substantially rebuilt and extended over centuries, remains active as part of Rome’s modern sewer and stormwater system. It is one of the oldest pieces of civil engineering in continuous use anywhere in the world.

Rome did not conquer its empire and then build roads to administer it. The roads and the conquest advanced together, each enabling the other in a feedback loop that eventually produced the most extensive road network the ancient world had ever seen. At its peak, the Roman road system covered somewhere between 250,000 and 400,000 kilometers, of which roughly 80,000 kilometers were stone-paved primary roads capable of moving legions, supplies, and official communications at speeds that would not be matched in Europe until the nineteenth century.