I seem to remember a tale of a french chap who went bust trying to bottle
this "earthscent". He went to his grave a pauper and never did manage it.

I think it may be microbes, fungi, bacteria etc. Dormant during dry spells
but bursting into life during damp periods. Emiting all kinds of gasses
during their respiration, digestion and division. They would love the warm,
damp conditions at the moment. Seeing as there are oil loving organisms too
as you mention tar ( a very natural substance BTW ) and the smell is very
noticable but different in the woods or countryside too.

I think a Copernic search for this french chap is in order. Perhaps it was
Lyall Watson in his book "Supernature" that mentioned it. That seems to ring
a bell.

Les