The earthy smell of geosmin
The scent is geosmin, an earthy-smelling compound made by certain soil microbes, including bacteria in the genus Streptomyces. When soil is disturbed or rain hits dry ground, tiny droplets and particles can carry geosmin into the air, where its musty, soil-like odor becomes noticeable. It is one contributor to the familiar smell associated with rain, though that scent is a mixture of compounds rather than geosmin alone.
Why a trace can stand out
The human nose is remarkably sensitive to geosmin: even a very small amount in air can register as an unmistakable earthy odor. That sensitivity makes sense as a useful warning system in nature: geosmin can signal damp soil and microbial activity. But the smell itself does not tell you exactly what is happening nearby, or whether the water or soil is safe.
The comparison with a great white shark tracking blood captures how striking this sensitivity is, but the two animals detect different chemicals in different environments. A shark follows dissolved molecules through water; a person notices geosmin carried through air. The next time rain releases that earthy smell, a trace of a microbe-made compound is enough to make an entire landscape seem newly vivid.
