is the scent of rain on dry earth. The word is constructed from Greek, petra, meaning stone + ichor, the fluid that flows in the veins of the gods in Greek mythology.
The term was coined in 1964 by two Australian researchers, Bear and Thomas, for an article in the journal Nature. In the article, the authors describe how the smell derives from an oil exuded by certain plants during dry periods, whereupon it is absorbed by clay-based soils and rocks. During rain, the oil is released into the air along with another compound, geosmin, producing the distinctive scent. In a follow-up paper, Bear and Thomas (1965) showed that the oil retards seed germination and early plant growth.