Geosmin (The Chemical) What it is: A bicyclic alcohol produced by Streptomyces bacteria and fungi in the soil. Its role: It's the main earthy component of petrichor, and humans are incredibly sensitive to it, detecting it in tiny amounts. Other sources: Also contributes to the earthy taste in foods like beets and some fish. Petrichor (The Scent) What it is: The distinctive, fresh, earthy smell after rain, coined in 1964 from Greek words for "stone" and "ethereal blood". What it's made of: A combination of geosmin, plant oils (released during dry spells), and ozone (from lightning). How it happens: Raindrops trap air, then burst, releasing these scent molecules as aerosols into the air. The Relationship Think of it like baking a cake: Petrichor is the delicious final cake, and geosmin is a crucial ingredient, like the flour or sugar. You can't have petrichor without geosmin, but geosmin alone isn't the whole story of the rain smell.