Warm earth, a freshly ploughed field. The smell of earth after the first rain in a while. The poetic name is Petrichor, the technical name is 'Giosmin'. There is a microbe which is released when warm dry earth is wetted. It's so distinctive and evocative because it can be tied down to one specific thing, first identified 100 years ago - the compound of Giosmin - so probably every human on earth will have a core aroma memory of the smell of rain. In fact it's one of the aroma compounds that humans can smell at incredibly low thresholds. 

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According to Brown University: “Geosmin, which literally translates to ‘earth smell’ was scientifically identified more than 100 years ago. In soil, bacteria produce the chemical compound. In water, blue-green algae make it. Along with the pleasant scent of warm, moist soil, geosmin is also responsible for the muddy ‘off’ taste in some drinking water".