When a dry spelling is broken by rain, it is a distinct smell in the air after the storm. The smell, a rich earthy smell that is very similar to the scent of fresh beetroot plugged from the earth, is called petrichor, a term coined by scientists in the 1960s to describe the afterglowing scent released by freshly moistened dry soil.  petrichor itself is caused by a very specific organic compound: geosmin. Geosmin is produced by many classes of microbes, including cyanobacteria and actinobacteria, and is released into the soil when these microorganisms die. Under the rain, the geosmin is released into the air along with a natural "additive" (an oil secreted by certain plants during dry periods and absorbed by clay-based soils and rocks).
Geosmin is also found in beet and bottom dwelling fish such as carp and catfish (and is responsible for the terrestrial / "muddy" taste of bottom-living freshwater fish). People are particularly sensitive to the compound and can smell in concentrations as low as five parts per trillion.