The smell of rain

What I don't know is a lot, but the list is one quantum shorter now that I've learned the word for the earthy smell left in the aftermath of a rainstorm: petrichor. 

This Smithsonian article describes the components that give rise to this scent. Of the primary components (plant oils, bacterial metabolites, and ozone, each of which has an interesting explanation in its own right) I was most intrigued by geosmin, which is produced by a certain soil-dwelling bacteria during spore production.

The human nose, it turns out, is incredibly, magnificently adapted to sensing geosmin - capable of detecting as little as 5 parts per trillion. This graphic on petrichor (from the great site Compound Interest) likens that concentration to a teaspoon in 200 Olympic size swimming pools. 

It raises the question of why humans have adapted to detect such minute amounts of a chemical that poses them no threat and confers no clear advantage. Perhaps the answer is related to the fact that humans generally like the smell of rain, and the ones who sought out the fertile conditions it heralds enjoyed a selective advantage. If so, it's just another way in which we have co-evolved with other life forms around us, many of them on the very edges of our senses.