The Trevi Fountain is a Baroque wall of water built against the whole front of a palace. You hear it before you see it, the roar bouncing off narrow lanes until the square opens and the wall pours down in front of you. Oceanus stands at the centre in a shell chariot drawn by two sea horses, one rearing and one held in check. Water the colour of jade slides over carved reefs into the basin below.
The water arrives through the Aqua Virgo, an aqueduct that Agrippa's engineers opened in 19 BC and Rome has never let run dry. Nicola Salvi won the commission in 1732 and shaped the theatre you see until his death in 1751; others finished it in 1762. The old ritual endures: a coin flicked over the shoulder to promise a return to Rome. The crowd is constant now and the ledges are off limits, yet the sound and the light on wet stone still hold everyone in place.