With the return of rain in France on Wednesday, a familiar scent—either beloved or disliked—should once again fill the air. This is **petrichor**, the distinctive aroma of wet earth that appears when rain falls on dry ground. The phenomenon is caused by a mix of molecules in the soil, which react when raindrops hit the ground. On Wednesday, most of France will be under rain, from Cherbourg to Nice, and from Biarritz to Metz. Almost no region will escape the return of precipitation, a welcome relief after weeks of drought. September was recorded as the "third least rainy September since measurements began in 1958," with an average precipitation deficit of 70% across the country, according to **Météo-France**. After the rain touches these dry soils, the scent of petrichor should be noticeable to everyone. This slightly woody, earthy smell has a name that comes from ancient Greek, meaning "blood of stone." The term was coined in 1964 by two Australian scientists: chemist Isabel Joy Bear and mineralogist Roderick G. Thomas. They used it to describe the scent that arises after a rain shower. Petrichor is caused by several molecules that accumulate in the soil when it is dry. One of these is a vegetable oil secreted by plants during hot weather, which is released when rain begins to fall. This oil then mixes with a volatile molecule called geosmin, which plays a central role in the scent. Geosmin is produced by microorganisms in the soil and is responsible for the earthy aroma. These two substances then combine with ozone, creating a strong scent that spreads through the air. The scent of petrichor can be pleasant for some and unpleasant for others, and it is typically stronger in the countryside than in urban areas. The human sense of smell is especially sensitive to this aroma. According to **France Info**, the human nose can detect geosmin at concentrations up to 1,000 times lower than those of many other perfumes, highlighting how potent and detectable this scent can be.