ALT Humans are extraordinarily sensitive to the smell of rain, known as petrichor, primarily due to the compound geosmin. We can detect geosmin at parts-per-trillion levels, whereas sharks typically detect blood in water at parts-per-million or parts-per-billion concentrations. This means humans are roughly 200,000 times more sensitive to the scent of rain than sharks are to blood. Such heightened sensitivity likely evolved because smelling rain helped early humans locate fresh water and fertile environments. What seems like a pleasant aroma today was once a crucial survival signal.