The fountain was designed to show off the acqueduct of the Acqua Vergine built by Marco Vipsiano Agrippa in 19 b.C. to supply water to the thermal baths which he built close to the Pantheon.
The water was named Vergin after the legend telling of a young girl who showed the original spring to a group of thirsty Roman soldiers. The first fountain to take the waters of the Acqua Vergine was built in 1453 for pope Nicholas V, designed by Giovan Battista Alberti in the spot called "of the Trejo" and through the years it took the name of Trevi.
The fountain marked an important turn point for the town which for centuries had to use water taken from the Tiber river. Three centuries later pope Clement XII decided to substitute the old fountain and instigated a competition amongst the best sculptors of his time to come up with something better. His aim was that to supply Rome with as much drinking water as possible and at the same time to give to the city a grandiose work of art.
Among the sketches was chosen that of the Roman Nicolò Salvi. The construction of the fountain lasted 23 years and it forms the east wing of the Poli Palace. It was modelled on the ancient arch of triumph crowned by the coat of arms of Clement XII. The figure of Ocean (Neptune) dominates proceeding, supported by tritons to either side; the one on the left struggling to control his horse represents a stormy sea, his partner on the right, blowing into a counch shell, symbolises the ocean in repose.
The statues in niches either side of Neptune are allegories of Health and Abundance, overseen by figures on the pediment who represent the four seasons. The relief on the fountain to the right of Oceanus illustrates the story of the Vergin which shows the spring to the Roman soldiers. On the other side Agrippa shows his project to the emperor. Into the basin, which represents the sea, tourists throw a coin to ensure their return to Rome. Another romantic rite is linked to the small fountain to the left side, called "small fountain of the lovers". According to the legend the couples who drink at its water will be faithful for ever.