The centre of the Piazza Navona is dominated by the Fountain of the Rivers - Bernini which was completed in 1651 and is one of the finest examples of Baroque architecture in Rome. The 4 rivers represent the 4 corners of Christianity and the 4 corners of the known civilised world which are The Danube (Europe), The Ganges (Asia), The Nile (Africa) and The Plate (Americas).
The obelisk that is in the centre of the fountain was brought to Rome from Egypt by Emperor Maxentius in the early 300s and was formerly housed at his circus, the Circus Maxentius, it disappeared for about 1300 years and then showed up on the Via Appia Antica. It was brought to Piazza Navona in the 1600s to decorate this fountain. Under the Obelisk is a huge empty grotto which is a much studied architectural wonder in Rome. The animals and plants which represent the four rivers look a little bit weird, believe it or not the animal for Africa is supposedly a crocodile! The reason for this is that Bernini had never seen any of these animals so he would have sculpted from information and bad sketches drawn from other people.