Always in neighborhood of the Cavern of San Giovanni d'Antro, along the path below the little fountain, you can see, on the wall, this strange artifact, consists of many pentagons joined together.
Initially I thought it was a common decorative stone, but then, look this perfect work in bas-relief, and undestanding the difficult to implement, I deduced this one should be a very important artefact.
Geometric shape of the Pentagon, here represented in repetitive mode and almost obsessive, does not absolutely part of Christian culture, which is totally absent.
The symbol of the Pentagon, is always considered a typical representation Heretic and Satanist, connected directly to the symbol of the Pentacle, because this containing a pentagon inside and outside.

I do not want to dwell much on the meanings that are attributed to this symbol, as it would go into the world of Esotericism, an argument devoid of objective, scientific evidence, but made primarily of beliefs.
What I can say with certainty is that in the whole wide symbology Christian, the Pentagon is a completely absent.
This fact shows that certain symbols have been "censored" by Christianity as being too antagonistic and dangerous.
Learn more about this symbol, on the Internet I came across several sites that bind directly the Pentagon to the Templars, and later to the Masonry movement.
Is even reported that the Templars culture, also to have adopted this symbols, was attacked and destroyed by the Christianity.
This engraving in stone, only confirms again that this Cavern was certainly a place of pagan pratice, then reconverted to Christianity, becoming a church, during Medieval age.