Beneath the papal altar in St. Peter’s Basilica in Rome, among the grottos and tombs, is the chapel of the Madonna della Bocciata. Inspiring. Revered. But often overlooked. Few of the five million annual Vatican visitors venture down the steps to tour the grottos, and of those that do, many miss the fresco of the Madonna placed there in the 1600s. Painted by Pietro Cavallini (c. 1250-1330)…