While in the Upper Garden, one cannot help but notice the Square Ponds - Western and Eastern. Their creation was solely driven by practical necessity. Initially, the ponds had no decorative elements and were considered only as reservoirs for water used by the Grand Cascade of the Lower Park. According to the project and estimate prepared by architect Jean-Baptiste Le Blond in 1718, the digging of the Square Ponds took place between 1719 and 1721. All work was carried out according to the decree of Peter I, who personally chose the shape of the ponds and their location. Their lower parts were finished by master Cardasie with tile stone, and the walls, one and a half bricks thick, were built up to a height of 2.5 meters. Despite their name, the pools are not strictly square in shape: their dimensions are 54 by 45 meters.

In 1737, gilded lead sculptural compositions created from models by B. K. Rastrelli were placed in them. The Western Pond was decorated with the group "Bathing of Diana," depicting the goddess of the hunt surrounded by nymphs, dogs, and dolphins. In the Eastern Pond, figures of the river god Alpheus and the nymph Arethusa were installed, surrounded by sirens and dolphins. The ponds became fountain systems in 1737 thanks to the developments of architects Blanc, Davydov, and the work of master hydraulic engineer Sualem.

It was then that the Western Pond received the gilded lead sculptural composition "Bathing of Diana," cast from models by Bartolomeo Rastrelli. This is a whole mythological scene, in which, besides Diana against the backdrop of a tree, one could see figures of nymphs, dolphins, and dogs. In the Eastern Pond, a multi-figure group "Arethusa and Alpheus" was installed, where the river god and the nymph are accompanied by 4 sirens and 9 dolphins. A couple of decades later, the Square Ponds were decorated with openwork gilded grilles. They remained in this form until the 1760s, when some figures had to be dismantled due to deterioration. Only 6 dolphins remained in each pool, which, according to architect Yakovlev’s plan, were to be part of the new decorative design of the ponds. Between 1770 and 1773, each fountain composition featured a central high jet, around which dolphins were arranged in a circle, spouting arched streams of water from their mouths.

The next change in the appearance of the fountains dates back to 1929. It was then, during restoration, that it was decided to somewhat recreate the appearance of the 1737 sculptural group, and in the center of the ring pedestals, the place of the central jet was taken by 18th-century marble statues "Spring" and "Summer."

By the end of the 18th century, the lead sculptures had lost their former shine, and only six gilded "water-acting" dolphins remained in each pond. At the beginning of the 20th century, the ponds' basins were decorated with 18th-century marble sculptures "Spring" and "Summer."

During the war, the Square Ponds suffered serious damage at the hands of the Germans: they not only removed the dolphins but also blew up the ponds' regulating mechanism. The basins were restored in 1946, and newly cast bronze dolphins, made according to samples found in the archives, took their place in 1957. Ten years later, a statue of the young Apollo appeared in the Eastern Pond, and in the Western Pond - Venus Italica. These marble copies of originals from the schools of Praxiteles and Canova remain the dominant elements of the Square Ponds to this day.

Sources:

https://peterburg.center/maps/petergof-verhniy-sad-fontany-kvadratnyh-prudov.html

https://peterhofmuseum.ru/objects/peterhof/fontani_kvadratnih_prudov