That distinctive earthy scent just before or after rain — many people call it "the smell of soil," but it isn't actually the smell of soil at all. This scent has a proper name, "petrichor," and behind it is a mix of soil bacteria, plants, and raindrop physics.
In 1964, CSIRO researchers Isabel Joy Bear and Roderick Thomas coined the term "petrichor" in a paper published in the journal Nature describing this scent. It combines the Greek word "petra," meaning stone, with "ichor," the golden blood said to flow through the veins of the gods in Greek mythology.
What we perceive as an "earthy smell" isn't soil itself, but the scent of geosmin, a compound produced by actinobacteria (mainly of the genus Streptomyces) living in the soil during their metabolic process. The human nose is extremely sensitive to geosmin — even a tiny amount dispersed into the air is easily noticeable.
According to Bear and Thomas's paper, oils released by certain plants during dry periods get absorbed into clay soil and rock. When rain falls and raindrops strike dry ground, the impact disperses tiny particles containing geosmin and these plant oils into the air as an aerosol. Add the smell of ozone produced by lightning, and you get the refreshing yet earthy scent distinctive to rainy days.
The longer a dry spell lasts, the more geosmin and plant oils accumulate in the soil. So the first rain after a long dry stretch often carries an especially strong petrichor scent. Conversely, during periods of frequent rain, there isn't enough time for these compounds to build up in the soil, so the scent tends to feel comparatively faint.
Petrichor is often detected right before or after rain falls, so it's not a reliable way to predict rain on its own. Saving the places you check often in sanCheck and checking rain probability ahead of time is more useful for planning your outings.