We also visited a 2000 year old aqueduct at Pont du Gard outside of Nimes, France. This was the highest of all Roman aqueducts at roughly 160 feet and was about 50 km long. The trough that held the water at the top was covered to keep it clean and fresh upon entering people's homes. It's made of limestone and had a gradient of about 1 inch every 50 feet - that's amazing engineering for the time period. This aqueduct was probably in use for about 500 years.
We hung out here and explored for a couple of hours before heading on to Avignon.