The Piazza Navona is exceptionally long and owes its shape to the ruins that formed it, for under the buildings that surround Piazza Navona are the remains of the Circus Domitianus, Domitian's stadium. In ancient times the stadium was the site of the Agonal games, from which the present piazza takes its name by corruption from "in agone" to "n'agone" to "navone" and finally "navona". - Roma Interactive
The basin of the fountain of Neptune was originally sculpted in 1576 by Giacomo della Porta. The figural statues were added three centuries later.