MargenThat smell of wet earth that rises when it starts to rain has a name of its own: petrichor. Two Australian researchers coined it in 1964, in the journal Nature, joining two Greek words: petra (stone) and ichor, the blood of the gods.
The main ingredient is geosmin, a molecule made by certain soil bacteria. When the drops strike the dry ground, they fling it into the air in tiny aerosols, and the wind spreads it, announcing the storm.
The extraordinary part is your nose: human smell detects geosmin at concentrations of parts per trillion — proportionally, we're more sensitive to it than a shark is to blood. Some scientists believe it's an inheritance from our ancestors, for whom smelling distant rain meant finding water.
That's why no one is indifferent to that smell: we've been celebrating it for millennia.
Source: Nature · CSIRO