When the miracle occurred at Mont-St-Michel, in 708, the whole area, which is now a bay, was a forest.
Two rocky peaks broke the massive forest. One was and is still called Tombelaine and the other was known as Mont Tombe. Mont Tombe, or as it is now called Mont-St-Michel, was once a Shrine to Belen, the Celtic god of Light.
It later became a Shrine to the Roman Mercury.
It did not become a Christian place of worship until the 5th century, when it was occupied by hermits from the other side of the channel (somewhere in the British Isles, as it is now known, probably from Ireland). They set up two oratories under the protection of Saints Stephen and Symphorien.
In 708 A.D. the Archangel appeared to St. Aubert who was then Bishop of Avranches.
Michael commanded that a Sanctuary be built on Mont Tombe, in his honor. And so, the Sanctuary was built on the very mount where the pagans worshiped.
This is not unusual in the history of our Church.
In Assisi, for example, there is a church called Santa Maria Sopra Minerva (Saint Mary over the temple of Minerva).
It is fitting for the Church to do this. Did not Saint Paul go to the pagan Gentiles and bring the Good News of Jesus to them, very often using their pagan beliefs to help them understand that the god, they believed in, was no match for the One and Only God.
When you visit the Shrine of Mont-St-Michel, you have to stop and wonder how they were able to do it, first to fight the strong pagan influence that still prevailed and then the physical difficulty?
But when God is in charge "cannot a child, with the Angel’s aid, cause the monuments of a pagan cult to tremble?"