geosmin

A chemical substance produced by the bacterium Streptomyces coelicolor and some cyanobacteria and fungi present in soil, which is released into the air when the earth becomes moist. It is mainly responsible for the characteristic smell of rain when it falls on dry ground. The impact of the drops against it causes the incorporation into the atmosphere of the odorous substances we perceive. The term comes from ancient Greek and means "arome of the earth."