The Roman Aqueduct in Segovia, Spain, has to be one of the most famous ones in the world. Every time I’ve seen it, I’ve been impressed. These photos aren’t the best, but they give a good impression of the how the structure spans the little valley there leading to the old part of town.
Moving closer, with some people in the photo, you get a better sense of the scale of it:
For more than a thousand years, this aqueduct brought water into town. That’s impressive for any piece of infrastructure. The Romans knew what they were doing, and weren’t afraid to do it well.
The Bent Page 
Granite block placement and proper elevation for I forget how many miles was unbelievably accurate ,