Why is Saint Peter's Basilica considered one of the holiest places of Christianity?

St. Peter's Basilica stands on the traditional site where Peter - the apostle who is considered the first pope - was crucified and buried. St. Peter's tomb is under the main altar and many other popes are buried in the basilica as well. Originally founded by Constantine in 324, St. Peter's Basilica was rebuilt in the 16th century by Renaissance masters including Bramante, Michelangelo and Bernini.