Circus of Nero (or Circus of Gaius (Caligula)) was a circus in ancient Rome placed at the location of today's Basilica of St. Peter in Vatican. All that is left today of this circus is obelisk that stood at its center.
Caligula (31 August 12 AD - 22 January 41 AD), a Roman emperor, began construction of this circus in the year 40 AD on the land of his mother, Agrippina. Claudius, who succeeded him, finished construction. Grimaldi says that the circus was 90 meters wide and 161 long. It was a place where Caligula and Nero trained racing with four horse chariots. In 65 AD, the first fist public persecution of Christians happened in this circus and Christian tradition says that Saint Peter lost his life there two years later. Saint Peter's tomb is in this area, in the cemetery near where the Circus was. Obelisk that stood in the center was placed there by Caligula. It was later (in 16th century) moved to Saint Peter's Square by the architect Domenico Fontana.
Circus was abandoned by the middle of the 2nd century AD so Constantine built the first basilica (Old St. Peter) at the site of the Circus using some of the existing structure. Most of the ruins of the Circus survived until mid-15th century. They were finally destroyed to make a space for the construction of the new St. Peter's Basilica.