Historia Eustachio Mariana

It describes his chance discovery of a ruined shrine of the Virgin Mary at Mentorella, the site where tradition held that the Roman martyr Saint Eustace had experienced conversion to Christianity.

I recognized a hidden hint from the Mother of God, put aside all other studies, and was by no means idle, but started the story of this holy place under the title: "Historia Eustachio-Mariana".

[5]Kircher began research to find the origins of the ruins he had discovered, and concluded that the shrine had been built at the time of Constantine the Great and commemorated the conversion of the Roman soldier Placidus, known to posterity as Saint Eustance.

[7] Having rebuilt and restored the chapel, Kircher added a device of his own design in the form of a set of large speaking trumpets that pointed at various villages in the valley below the shrine.

[2][4]: 53, 125 The book also included illustrations of the wall-paintings he found in the ruined shrine, as well as some of the treasures that had formerly belonged to it, kept in safekeeping by the Duke of Polana in his library.

These included a medieval silver cross, a large bronze candlestick, and an oak tablet carved with the legend of Saint Eustace.