This excerpt of Ferdinand Gregorovius' book Wanderjahre in Italien describes Arcangel Michael's apparitions on the Gragano.
A rich man named Garganus once lived in Sipontum. His herds were grazing in the mountains. One day a beautiful bull disappeared, and he searched for it with his herdsmen in all gorges of the mountain ridge. As time went by he got in rage about not finding the animal. Finally he found the bull at the entrance of a cave, and in his rage about the long search he shot at it. But the arrow returned and hit the shooter.
This was considered a miracle and reported to the bishop Laurentius in Sipontum. He ordered three days of fastening, and at the third day, the 8th May 493, he had an apparition of Archangel Michael, who told him that the grotto was consecrated as a church by himself and should be a place of worship to his and the other angels honour in the future. But he had to reappear various times to convince the procrastinative bishop to finally visit the cave with other believers. Saint Michael also appeared the Sipontians and saved them in a battle against pagans, which besieged the city.
When the christs entered the cave, they found it lighted by a heavenly light and transformed into a chapel by the hands of angels. In the rock face a purple covered altar was erected. Laurentius built a church in the entrance of the grotto and consecrated it with the authorization of the pope Gelasius on 29th September 493 to the archangel.