Piazza Navona started out as a stadium named the Circus Agonalis, or the Stadium of Domitian, commissioned by the emperor Domitian in 80 AD and dedicated in 86 AD. It was built as a gift to the people of Rome, and was used mostly for athletic contests. It was Rome's first permanent venue for competitive athletics. It was an intimate stadium seating 15,000 or 20,000 people, a more appropriate venue for foot-races than the much larger Circus Maximus where chariot races were held. The Piazza Navona sits over the interior arena of the stadium. The buildings now at the outer edge of the piazza incorporate the stadium's original lower arcades. Remnants of the original stadium can be seen today, although they were being renovated when I visited and saw them in fall of 2013. Walk out the northern end (the curved end) of Piazza Navona and turn left. Then on your left side are a couple places where you can peek underground at the foundations of the stadium. During the fall of 2013, a very kind worker saw me trying to peek through the renovation covers and allowed me access to a place where I could look down on the renovation (all except the 2nd photo below).

                             

In a subsequent visit to Rome during October of 2014, the underground excavation and conversion into a museum was complete, however when I walked past it during the morning, it wasn't yet open. Still, here are 3 photos of what's visible when looking down from the street.

              

And a few hours later, the museum, which features the architecture of the building itself as well as serving as a military and gladiator museum, was open and I visited.

                                                                                                                                                               

See also:

• My Circus Agonalis photos