Ancient tradition asserts that the first basilica was constructed over the ancient tomb of the apostle Peter, who was martyred in the nearby Circus of Nero on an inverted cross in the first century AD. After Constantine converted to Christianity he began a great basilica on the site in 324, which had previously been an ancient cemetery in which both pagans and Christians were buried. This structure remained until the 15th century when it was decided that a new, more magnificent structure should replace it. Construction actually began under Pope Julius II (of the Agony and the Ecstasy) in 1506, but it wasn't completed until 1615 under Pope Paul V. Many famous artists worked on the various phases, including Michelangelo, who designed the dome (later greatly reworked), and Bernini, who designed the great colonnaded St. Peter's Square, where many thousands gather on important events and also the magnificent bronze canopy (which is comprised of 927 tons of dark bronze, taken from the roof of the Pantheon in 1633) over the papal altar and the relics of Peter the Apostle. It also fills the great vertical space under the great dome. The marble opus sectile floors seen here may have originally been from the ruins of a Roman public building.