Le Mystère des Cathédrales ("The Mystery of the Cathedrals"), first published in 1926 by the enigmatic alchemist Fulcanelli, presents the thesis that Gothic cathedrals are not merely religious structures but massive, encrypted repositories of medieval alchemical knowledge. Fulcanelli argues that these buildings serve as "stone books" that hold the secrets of the Great Work—the transmutation of matter and the self—hidden in plain sight through symbolism. Le Mystère des Cathédrales functions as a guide to reading the "language" of gothic architecture, demanding that the reader have knowledge of alchemical principles and Gothic art to understand its full meaning.