Geosmin

Today's fish tasted muddy, not because it is bred in sewage-water (which is) but because of an organic compound called geosmin. This smelly molecule is produced by my favorite mold of the order Actinomycetales, and released when these microorganisms die. 

Everyone knows that a few drops of lemon juice on the fish neutralizes the muddy odor. The molecule breaks down in acidic media.

The human nose is extremely sensitive to geosmin and is able to detect it at concentrations as low as 5 parts per trillion. This had important evolutionary impact as it helped to locate fresh pastures after a period of drought. I rather love this earth smell after rain.