I took these photos a few months ago. This huge fountain is visible from Trastevere and other parts of the city. It was built in the 1600s with the basin being added in 1690 by Carlo Fontana (really).
This pool of fresh water is from an ancient aqueduct built in AD 109. Don't think about taking a dip though. There are armed men and women with machines guns standing around. Security is pretty heavy due to all the embassies in the area.