Fontana dell'Acqua Paola

Listen first. Before you look at anything, there is the sound — a steady, heavy rush of water falling into a wide stone basin. It does not trickle or splash like the small fountains in the streets below. It roars, softly, all day and all night. Feel the slope of the road under your feet, too. You have climbed the Janiculum Hill, and the air up here is a little cooler and a little quieter than the city below. In front of you stands a wall of white stone, carved like a triumphal arch.

This is the Fontana dell'Acqua Paola. Romans have a simpler name for it — Il Fontanone, which means the big fountain. The nickname fits. The structure looks less like a fountain and more like the front of a church, or a gateway for a returning emperor. It has five openings, with the three central arches larger than the two at the sides. Water pours out from under them into the basin. Take a moment to step back and see the whole front at once. It was designed to be seen exactly like this.

The fountain was built in the early seventeenth century, between 1610 and 1612. It was commissioned by Pope Paul the Fifth, who was born Camillo Borghese. That is where the name comes from. Acqua Paola simply means Paul's water. The design is credited to two architects, Giovanni Fontana and Flaminio Ponzio. Look up at the top of the structure. The large stone tablet carries a Latin inscription that names the pope and announces his gift of water to the city. Popes in this period liked their names written large.

To understand why this monument exists, you need to think about water. Ancient Rome was fed by a network of aqueducts — long channels that carried water from springs and lakes across many kilometres of countryside. After the fall of the empire, most of these channels broke down. For centuries, much of the city depended on wells and on the river. Restoring an aqueduct was one of the greatest public works a pope could offer. And every restored aqueduct needed a grand finish — a showpiece fountain where the water arrived.

Here is the first surprise. Paul the Fifth did not build his aqueduct from nothing. His engineers restored an ancient Roman one, the Aqua Traiana, built by the Emperor Trajan in the early second century. That channel brought water from the area around Lake Bracciano, north-west of Rome. So the water you hear falling into this basin travels along a route laid out by Roman engineers almost two thousand years ago. The pope repaired it, renamed it, and put his own name on the end of it.

The second surprise is in the stone itself. Much of the marble used for this fountain was not freshly quarried. It was taken from ancient ruins in the city. Material was stripped from the Roman Forum, including from the remains of a temple in the Forum of Nerva. Several of the granite columns came from the old Basilica of Saint Peter, which was being demolished at the time to make way for the new basilica. In other words, this wall is partly built from pieces of older Rome.

This kind of reuse was common in Renaissance and Baroque Rome. Builders saw the ancient ruins as a quarry, close at hand and full of fine stone. Today we might call it destruction. At the time, it was simply practical. Stand close to the columns and look at their surfaces. The granite has a different colour and texture from the bright white marble around it. That difference is one small sign that these columns had a long life before they arrived here.

Now look for the family symbols. The Borghese family used two heraldic animals — an eagle and a dragon. You will find them carved into the decoration of the fountain. Popes marked their buildings with their family arms, the way a company today puts its logo on a new headquarters. Once you know the eagle and the dragon, you will start to notice them on other Borghese projects across Rome. They are a kind of signature, repeated in stone throughout the city.

The fountain you see today is not exactly the one finished in 1612. Originally, the water fell into smaller basins. Later in the seventeenth century, the architect Carlo Fontana added the large, wide basin in front of you. That change gave the monument its calm mirror of water. On a still day, the reflection of the arches sits on the surface. The basin also adds a little distance between you and the façade, which makes the whole front feel even larger.

The water did more than look beautiful. From this high point, it flowed downhill to serve the neighbourhoods below, especially Trastevere, the old district between the hill and the river. Height mattered. Water moved by gravity, so a terminal fountain on a hill could feed a wide area. Locals have long said that the Acqua Paola was not the finest drinking water in Rome, since it came partly from lake sources. Whatever the truth of that reputation, its supply was valuable for everyday life.

This hill has seen other kinds of history too. In 1849, the Janiculum was the scene of heavy fighting. Defenders of the short-lived Roman Republic, led in part by Giuseppe Garibaldi, fought French troops on these slopes. The street in front of the fountain carries his name — Via Garibaldi. Further along the hill, you will find monuments and busts that remember those battles. The fountain stood through it all, still pouring water while the city changed around it.

In more recent years, the fountain found a new audience on screen. It appears in the opening scenes of the 2013 film La Grande Bellezza, The Great Beauty, by the director Paolo Sorrentino. Many visitors now come because they remember those images. You may also see wedding couples here, posing for photographs against the white stone. Turn around, too. Across the road, the terrace looks out over the rooftops and domes of Rome. On a clear day, the view stretches far across the city.

Before you leave, walk a short way downhill. The church of San Pietro in Montorio stands nearby. In its courtyard is the Tempietto, a small round temple by the Renaissance architect Donato Bramante. It is tiny compared with this fountain, but it had a huge influence on later architecture. The two monuments make a good pair — one quiet and perfect in miniature, the other loud and theatrical. Both show how much effort Rome put into marking sacred and public places.

Now, come back to the sound. Stand at the edge of the basin for a minute. Notice how the noise of the water covers the traffic and the voices around you. Look at the patches of darker stone, the moss near the waterline, the light moving across the arches. This water has run from the hills toward this spot since Trajan's time, with long gaps and a papal restoration. As you walk away, listen to how slowly the roar fades behind you.