Answer: Geosmin

When a dry spell is broken by rainfall, there is a distinct smell in the air after the storm. That smell, a rich earthy smell that strongly resembles the scent of fresh beets plucked from the soil, is called petrichor—a term coined by researchers in the 1960s to describe the post-rainfall scent released by freshly dampened dry earth.

The petrichor itself is caused by a very specific organic compound: geosmin. Geosmin is produced by many classes of microbes, including cyanobacteria and actinobacteria, and released into the soil when these microorganisms die. During rainfall, the geosmin is released into the air along with a natural “additive” (an oil that is exuded by certain plants during dry periods and absorbed by clay-based soils and rocks).

Geosmin is also found in beets and in bottom-dwelling fish like carp and catfish (and is responsible for the earthy/”muddy” flavor of bottom-dwelling fresh water fish). Humans are particularly sensitive to the compound and can smell it in concentrations as low as five parts per trillion.