Before you throw the coin — and you should — look at the water for a second. It's older than almost anything you've seen in Rome today.
It's the end of a 2,000-year-old pipe
This whole fountain is, really, the end of a pipe. A very grand pipe. Behind the marble, the Trevi is fed by the Acqua Vergine — an aqueduct that Agrippa finished in 19 BC, under the emperor Augustus. Around twenty kilometres of channel, most of it underground, still carrying water to this exact spot more than two thousand years later.
The fountain itself is just the showpiece the Romans built where the water arrives. They had a word for that: a mostra — a showing-off.
The girl who named the water
The name means "the virgin water", and there's a girl to thank for it. The story goes that a young Roman girl led a band of thirsty soldiers to the hidden spring that still feeds the aqueduct. Look to the relief on the right and you can find her, pointing the soldiers to the source. She's the reason the water has a name.
Now, the coin
The ritual is fussy and specific: right hand, over your left shoulder, your back to the fountain. One coin, and you'll come back to Rome. Two, and you'll fall in love here. Three, and you'll get married.
People throw in around three thousand euros a day, and none of it stays. Every morning it's collected and given to a charity that runs food banks and shelters across the city.
One last thing before you go. The giant in the middle isn't Neptune, though everyone calls him that. It's Oceanus — an older, stranger god, the river the Romans believed circled the entire world. The two horses hauling his shell are the sea in two moods: one rearing and furious, one calm. And Salvi, the man who designed all of it, spent thirty years here and died before it was done. He never heard the water run.
So make your wish. Then walk five minutes to the Pantheon — the dome you're about to stand under has been the largest of its kind for nineteen hundred years.