Aqueducts

It blows my mind to think that ancient Greek and Roman empires used aqueducts to move water into the city to supply public baths, latrines, fountains, and water to some private households. Aqueducts also provided water for mining operations, milling, farms, and gardens.
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Think about how hard it must have been from an engineering knowledge standpoint and the unsophisticated tools they had to use to accomplish such amazing feats. Maybe not as mind-blowing as how they built the Great Pyramids but I'm astounded as to their brilliance.