Geosmin

An organic compound responsible for giving beets an earthy taste as well as a contributor to the strong scent, known as petrichor, that occurs in the air when rain falls after a dry spell. It has a distinctly earthy flavor and aroma produced by a type of Actinobacteria.

Geosmin is considered an off-flavor in beer and is cause by contaminated brewing liquor, rinse liquor, or dilution liquor. It is caused by the growth of microorganisms in the water supply.  

Beers with geosmin present are typically described as tasting and smelling dirty or contaminated.