In A Word: Geosmin

geo·​smin jē-ˈō-smən

: a volatile, organic compound (C12H22O) that is formed especially by soil-dwelling bacteria (such as streptomyces) and aquatic cyanobacteria and that may contribute to the earthy, pleasant odor of petrichor or impart a disagreeable, musty taste and odor to drinking water and certain fish

It’s not often that we humans outcompete our fellow animals when it comes to scent detection. A bear can smell a greasy grill from more than 20 miles away, a shark can sniff out blood a quarter of a mile off, and even your housecat can detect the scent of you when you’re about a mile away, though that doesn’t mean it’s the least bit interested in seeking you out. 

But there’s one scent in particular that humans excel at detecting. It’s called “geosmin,” and it’s the smell of rain and soil, something we can detect at as low as 100 parts per trillion. 

It’s fair to assume that being able to detect geosmin in such minute amounts might be key to our survival. When we all lived off the land, such a skill helped us locate drinking water and fertile soil for crops. As Cass Marketos wrote in The Rot, her newsletter about compost, “In that sense, geosmin doesn’t just signal ‘earth’ to us, then, but specifically good earth. It means healthy, rich soil — teeming with life, invisible but deeply felt, threaded all the way into the oldest parts of our molecular formation.”

In our urbanized lives, where crops are purchased at the grocery store, where water comes in plastic, it’s easy to forget that it wasn’t always thus, that our survival was — is! — woven into and dependent upon the health of the world around us. Smell works thanks to olfactory sensory neurons — actual “brain” cells situated in the top of your nose. Those neurons tell us what we’re smelling, which, in the case of geosmin, is … earth. Or, more to the point, life. No wonder we have superpowers when it comes to sniffing it out.

“Fall,” Cass Marketos writes, “proffers all the necessary ingredients for this odor to become abundant. The leaves come down and begin their long, slow process of decomposition. The soil enrichens. The rain falls. The earth retreats into a period of generative and active breakdown. Everywhere, everywhere — the smell of earth.”