Think about how challenging 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.
Note: Below is a Roman aqueduct. It went from this to the small pipelines that you and I have going into our homes. My mind isn't creative enough to be able to visualize how they built these systems without all the construction equipment we have today. Amazing!