Art experts had not known of the existence of a centuries-old sketch believed to be Michelangelo's work before it was found by chance by a researcher going through the archives of St. Peter's Basilica, a Vatican official said Friday. The existence of the sketch was "up to now unknown," said Maria Cristina Carlo-Stella, who runs the Fabbrica di San Pietro, the office where the archives are kept. A researcher "happened upon this little sketch, which he believes is by the hand of Michelangelo," she told The Associated Press. The discovery was reported Thursday in the Vatican's daily newspaper, L'Osservatore Romano, which said the sketch for the dome of the basilica was done for stonecutters working on its construction. The report said it was believed to have been done in 1563, less than a year before Michelangelo's death.