Why does it smell so good right before and after it rains?

It starts to rain on dry ground and that unmistakable smell of wet earth appears, fresh and pleasant. It has a name — petrichor — and a surprising backstory: that aroma isn’t made by the rain.

The star ingredient is a molecule called geosmin, and what produces it is bacteria living in the soil (mostly of the genus Streptomyces). When the ground is dry, geosmin builds up. As the first drops fall, they strike the soil and fling thousands of microscopic particles loaded with geosmin into the air, which the wind spreads and your nose catches.

And here’s the astonishing part: your sense of smell is extraordinarily sensitive to geosmin. You can detect it at tiny concentrations, better even than many animals. It’s thought to be a useful inheritance: for our ancestors, smelling water and fertile soil from a distance could be a matter of survival.

The smell of rain isn’t from the rain: it’s the breath of millions of soil bacteria, and your nose is tuned not to miss it.