The Acqua Paola Fountain, better known as the "Fontanone," is a monumental fountain located on the Janiculum Hill.
It was built between 1610 and 1614 at the behest of Pope Paul V as the terminal point of the Trajan Aqueduct, constructed by Emperor Trajan in 109 AD and originating from Lake Bracciano to supply the Trastevere neighborhood.

The construction of the fountain was entrusted to the architects Giovanni Fontana and Flaminio Ponzio, who, at the pontiff's request, were inspired by a triumphal arch. The fountain consists of five arches, three larger and two smaller, flanked by columns, four of which are in red granite from the façade of the old St. Peter's Basilica. The columns support the architrave which contains the following inscription: "ANNO DOMINI MDCXII PONTIFICATUS SUI SEPTIMO", i.e. "In the year of the Lord 1612 during his seventh Pontificate".

The large and beautiful epigraph in the attic, which reads: "PAULUS QUINTUS PONTIFEX MAXIMUS AQUAM IN AGRO BRACCIANENSIS SALUBERRIMIS E FONTIBUS COLLECTAM VETERIBUS AQUAE ALSIETINAE DUCTIBUS RESTITUTIS NOVISQUE ADDITIS XXXV AB MILLIARIO DUXIT", however contains an inaccuracy: the restoration is in fact cited of the Alsietino aqueduct (AQUAE ALSIETINAE) coming from the "lacus Alsietinus" (now Lake Martignano), while in reality it was the ancient aqua Traiana coming from Lake Bracciano that was restored.

At the end of In the seventeenth century, the architect Carlo Fontana (1638-1714), at the behest of Alexander VIII, modified the facade, giving the fountain its current form: a large and magnificent hemicycle-shaped white marble basin was added to replace the five pre-existing collection basins.