system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes that indescribable scent when it first starts to rain?
gpt
It’s called “petrichor”, and it’s a scent that’s created when rain hits dried soil. Plant oils soak into absorbent dirt over time, and are then released into the air when the dirt starts getting wet. 

The scent has been isolated and is commercially available as a fragrance material called “geosmin”. The human sense of smell is *ridiculously* sensitive to geosmin, and you can smell it in concentrations as low as just a few parts per *trillion* - one single drop of geosmin in an Olympic-size swimming pool is detectable.