HAVE YOU EVER WONDERED , WHAT CAUSES A MUDDY EARTHY SMELL AFTER RAIN....???     

Answer ...GEOSMIN

Geosmin is an organic compound with a distinct earthy flavor and aroma produced by a type of Actinobacteria, and is responsible for the strong scent that occurs in the air when rain falls after a dry spell of weather.
Geosmin is produced by the gram-positive bacteria Streptomyces, a genus of Actinobacteria in the order Actinomycetales, and released when these microorganisms die. Communities whose water supplies depend on surface water can periodically experience episodes of unpleasant-tasting water when a sharp drop in the population of these bacteria releases geosmin into the local water supply. Under acidic conditions, geosmin decomposes into odorless substances.
The human nose is extremely sensitive to geosmin and is able to detect it at concentrations as low as 5 parts per trillion.
Geosmin is responsible for the muddy smell in many commercially important freshwater fish such as carp and catfish.