Geosmin smells of earth:
You can smell vanishingly small amounts of it, too – mere nanograms of the stuff! It smells this way for a reason – countless soil bacteria busily produce it in your backyard as you read this.
The first thing I thought of when I started this entry was Demeter’s Dirt fragrance. I imagine this has to have some of the stuff in it – I’ve only ever smelled it briefly in Whole Foods, though.
I love smell because I never fail to marvel at the fact that a pure single molecule can exhibit such complex aromas. My favorite example is acetophenone.