Segovia's Roman Aqueduct
I was in Segovia recently as part of a short city break to Spain.
One of the most impressive sights is the ancient Roman Aqueduct dating from the 2nd century BC which still delivers drinking water into the city to this day.
And a remarkable feature of the aqueduct is that its large stones are held in place by their own weight and Roman engineering, rather than cement or mortar.
Which reminded me of the old Monty Python sketch on the subject of "What have the Romans ever done for us?"