Rain and smell...

Everyone who has seen raining would have noticed this. It will smell immediately after the rain starts falling.
The reason for this peculiar smell that appears immediately after the rain shower is due to some volatile chemical compounds that are released by soil inhabiting bacteria which are named as streptomcetes.
These streptomcetes are abundant in dry warm soil. Millions of them can be detected in a pinch of soil. They release some compounds called geosmin and methyl isoborneol when wetted by water after a long dry spell.
That is why we get musty odor only after the first rain of rainy season or water splash on dry soil.