Your gaze is immediately drawn to the imposing Fountain of the Four Rivers in the center of the piazza, dominating the scene with its powerful presence and figures that seem to come alive from the sound of the rushing streams of water.
The four giant nudes that form the statue are the personification of the principal rivers of the continents known at the time: the Nile represents Africa, with its veiled head because the source of the river had yet to be discovered, the Ganges Asia, the Danube Europe and Rio de la Plata, the Americas.
The marble giants are arranged on a travertine shoal at the center of a scene of carved grottoes and decorated with flowers, exotic plants and animals. There are seven animals ranged around the fountain: a horse, a sea monster, a serpent, a dolphin, a crocodile, a lion and a dragon. Overhead, at the top of the obelisk, is a dove, symbol of the Holy Spirit and emblem of the pope that commissioned the fountain.
This marvelous work by Gian Lorenzo Bernini can also be interpreted as a spectacular metaphor: divine grace, symbolized by the Dove, rains upon the Earth and the 4 continents whose rivers feed the ocean, represented by the large pool. This was a theme extremely dear to the pope, whose emblem is placed several times on the fountain to remind us of his role as earthly intermediary of divine will.
Towering over the group of sculptures is a gigantic Egyptian obelisk; it's actually a Roman copy done during the reign of the emperor Domitian. The obelisk rises so high into the sky that, at the time, people were severely worried about the fountain's stability!
Also noteworthy was the manner in which he obtained the commission for the fountain: the Pamphilj Pope Innocent X had in fact already assigned the project to Borromini - at the time, the rivalry between the two architects was well known - but Bernini managed to get his hands on the project thanks to the good graces of the greedy and powerful sister-in-law of the pope, Donna Olimpia, known to the general population more for her miserable character than for her virtues. Bernini sent her a silver model of the Fountain; the pope just "happened" to see it and was so struck by it that he abandoned Borromini's design and commissioned the work to Bernini.
The people didn't miss their chance to spread urban legends regarding the fountain, fed also by the famous rivalry between the two artists. Thus it was believed that the statue of the Nile covered its head so as not to have to see the church of Sant'Agnese in Agone, designed by Borromini right in front of the Fountain; the Rio de la Plata seems to raise its hand to defend itself from a possible collapse of the church and the statue of Sant'Agnese on the facade of the church, with her hand on her chest, seems to reassure the Rio de la Plata of the church's stability; these are all obviously only nasty rumours since the fountain was built well BEFORE the church.