The actual carving of the sculpture was done by assistants to Bernini. Its travertine base, populated with four giant marble personifications of the rivers Danube, Nile, Ganges and Plate the four great rivers in the four continents recognized by the Renaissance geographers: the Nile in Africa, Ganges in Asia, Danube in Europe, and Río de la Plata in America. Each has animals and plants that further carry forth the identification.... supports an ancient granite obelisk moved from the Circus of Maxentius. Bernini daringly pierced the base on all sides, thus opening a view through and making the massive obelisk appear to hover weightlessly above the void.