4S,4aS,8aR)-4,8a-Dimethyl-1,2,3,4,5,6,7,8-octahydronaphthalen-4a-ol

There's a great post over at Metafilter on the above molecule, commonly known as Geosmin- responsible for the earthy smell after rain (one of my favourite smells of all time.). It's produced by microorganisms in the soil, such as the mycelial soil bacteria Streptomyces.

Humans can smell geosmin at 10 parts per trillion(!)- it's speculated that this sensitivity arose as an evolutionary advantage to help find drinking water.

Geosmin can also contaminate wine or water, making it taste musty. And it contributes to the flavour of the mighty beet, for which I am very thankful.  I love me some beets.