Pause for a moment and imagine the scent of rain. You can almost feel it in your nose.

There’s that bright, sweet smell on the wind before the rain comes. Then the scent of fresh earth and grass as the rain falls. And a damp, musty aroma like a forest floor that lingers afterward.

What gives rain these distinctive aromas? And why do we find them so memorable?

Well, the pre-rain smell is ozone. Lightning in the clouds splits nitrogen and oxygen gas into single atoms, which recombine into things like nitric acid and ozone.

Downdrafts and the first drops of rain carry ozone to the ground, where we experience it as a sweet, lightly acidic smell.

As the rain starts falling, drops of water strike plants and the ground and liberate organic compounds and aromatic oils, splashing them into the air as aerosols.

Once the soil and dead leaves on its surface become wet, bacteria begin to produce geosmin, an alcohol that’s the signature musty-basement smell of decaying plant matter.

Humans are incredibly sensitive to the smell of geosmin; we’re able to pick up just a few parts per trillion in the air.

And scientists think there’s a very good reason for this. Early humans depended on natural water sources. Those who could find water with their noses prospered. Those who couldn’t may not have survived to pass on their genes, and noses, to the next generation.