Few know that Piazza Navona is the "modern" look to an ancient Roman Empire stadium. The buildings surrounding the piazza sit atop the old stadium's original arcades. The stadium is Emperor Titus Flavius Dominitianus' gift to the Roman people, inaugurated in 86AD. The Stadium of Dominitian hosted foot games such as sports and the occasional gladiator fights, when the Colosseum was being repaired from a fire. The Stadium of Dominitian held around 20000 spectators and it's ruins can be visited today, with private tours. The original Stadium of Dominitian's arcades and supporting structure is preserved underground in remarkable splendor, so much so that it is the strong foundation of today's Piazza Navona AND a section of it is used for opera and theatrical performances. Rome's underground is absolutely incredible!