The source is an Internet Archive page for a book titled "Fulcanelli: Master Alchemist: Le Mystere des Cathedrales, Esoteric Interpretation of the Hermetic Symbols of The Great Work." The book claims that the legendary alchemist Fulcanelli left behind a manuscript in 1926 detailing the hidden alchemical symbolism found in Gothic cathedrals throughout Europe. The book argues that these symbols have been openly displayed for 700 years.

NOTE: If we find this is of some interest to the listeners of this podcast we may bring you more about this fascinating book over the coming podcasts.