As an engineer and an avid lover of architecture, in my best opinion, the greatest feat of engineering must be the aqueducts. The Romans have set a standard unparalleled for over a thousand years after the fall of Rome. Many cities still maintain and use the ancient aqueducts even up to today, although these open channels have been replaced by pipes.
The Romans typically built numerous aqueducts to serve any large city in their empire, as well as many small towns and industrial sites. The city of Rome itself, being the largest city then, had the largest concentration of aqueducts, with water being supplied by eleven aqueducts constructed over a period of 500 years. Scholars even used its water supple to predict the size of the city.
They served potable water and supplied the numerous baths and fountains in the city, as well as finally being emptied into the sewers, where they performed their last function in removing waste matter. The methods of construction are well described by Vitruvius in his work De Architectura written in the first century BC. His book would have been of great assistance to Frontinus, a general who was appointed in the late first century AD to administer the many aqueducts of Rome. He discovered a discrepancy between the intake and supply of water caused by illegal pipes inserted into the channels to divert the water, and reported on his efforts to improve and regulate the system to the emperor Nerva at the end of the first century AD. The report of his investigation is known as De aquaeductu.
Now you can find various aqueducts in many forms like a drain, pipes or even a canal. However, the ancient aqueducts are still a show of the love of design, painstaking effort and culture of a civilisation. They are the epitome of a great marriage in architecture and engineering.
Aqueduct of Segovia.