Located at the foot of Minervino, in a valley found at the end of a natural channel that once was a small river called “Matitani", the grotto of San Michele is a karstic cavern whose formation dates back to the Quaternary period (2 million years ago), a time when this area, which until that point had remained submerged, started rising above sea level.
The first written testimony regarding this grotto are found in a parchment kept in the Montecassino Abbey, and dated February 12 of the year 1000.
Most probably the grotto was already known and frequented before that, perhaps since Paleo-Christian times. 
As an ancient place to worship Saint Michael, the Grotto is of valuable historic, artistic and speleological interest.
Tradition has two dates to celebrate Saint Michael: September 29, with a feast dedicated by a Romanesque basilica to the cult of the Saint, and May 8, the feast to commemorate the apparition of the Archangel on Gargano.