Aqueduct of Trajan, Underfoot

This brass line sits in the Cryptoporticus (what some would call the basement) of the American Academy.

With a little lifting (two people are required), you can access a ladder, which takes you down to a segment of Trajan's aqueduct inaugurated in 109 CE. It ran 25 miles, engineered to have a steady slope over that entire length.  The water still flows (in the Acqua Paola, just down the hill from the Academy) but through new tunnels.