Penicillium roqueforti is the mold species that produces the blue-green veining in blue cheeses such as Roquefort, Gorgonzola, Stilton, and Danish blue. Cheesemakers grow the mold on rye bread or another starch substrate, then work the spores into the curd; during aging, the mold spreads along cracks and air pockets, creating the marbled veining and the sharp, pungent, salty flavor associated with blue cheese. A folk tale credits a shepherd who left bread and cheese in a cave with discovering Roquefort, but this is a traditional story, not a documented origin. The species thrives in cool, humid conditions, which is why blue cheeses are traditionally aged in caves.