The Virgin's Water

Everyone throws a coin. Almost nobody knows what they are throwing it into: a spring found by a girl two thousand years ago, carried into the city by an aqueduct that still runs, and finished with a fountain so big it took thirty years. A water-keeper leads you from the last visible arch of the aqueduct to the fountain, the coin, the films, and a church next door where the popes kept their hearts. About 30 minutes, five stops.