<P> The area now covered by the Vatican City had been a cemetery for some years before the Circus of Nero was built . It was a burial ground for the numerous executions in the Circus and contained many Christian burials, because for many years after the burial of Saint Peter many Christians chose to be buried near Peter . </P> <P> In 1939, in the reign of Pope Pius XII, 10 years of archaeological research began, under the crypt of the basilica, an area inaccessible since the 9th century . The excavations revealed the remains of shrines of different periods at different levels, from Clement VIII (1594) to Callixtus II (1123) and Gregory I (590--604), built over an aedicula containing fragments of bones that were folded in a tissue with gold decorations, tinted with the precious murex purple . Although it could not be determined with certainty that the bones were those of Peter, the rare vestments suggested a burial of great importance . On 23 December 1950, in his pre-Christmas radio broadcast to the world, Pope Pius XII announced the discovery of Saint Peter's tomb . </P> <P> Old St. Peter's Basilica was the 4th - century church begun by the Emperor Constantine the Great between 319 and 333 AD . It was of typical basilical form, a wide nave and two aisles on each side and an apsidal end, with the addition of a transept or bema, giving the building the shape of a tau cross . It was over 103.6 metres (340 ft) long, and the entrance was preceded by a large colonnaded atrium . This church had been built over the small shrine believed to mark the burial place of St. Peter . It contained a very large number of burials and memorials, including those of most of the popes from St. Peter to the 15th century . Like all of the earliest churches in Rome, both this church and its successor had the entrance to the east and the apse at the west end of the building . Since the construction of the current basilica, the name Old St. Peter's Basilica has been used for its predecessor to distinguish the two buildings . </P> <P> By the end of the 15th century, having been neglected during the period of the Avignon Papacy, the old basilica had fallen into disrepair . It appears that the first pope to consider rebuilding, or at least making radical changes was Pope Nicholas V (1447--55). He commissioned work on the old building from Leone Battista Alberti and Bernardo Rossellino and also had Rossellino design a plan for an entirely new basilica, or an extreme modification of the old . His reign was frustrated by political problems and when he died, little had been achieved . He had, however, ordered the demolition of the Colosseum and by the time of his death, 2,522 cartloads of stone had been transported for use in the new building . The foundations were completed for a new transept and choir to form a domed Latin cross with the preserved nave and side aisles of the old basilica . Some walls for the choir had also been built . </P>

When did construction commenced of st peter basilica