Humans are super-sensitive to it as well. In fact, according to plant scientists at the U.K.'s John Innes Centre, we can detect it in as few as 100 parts per trillion – that's "trillion" with a "T" – which, as they vividly put it, means we're better at smelling it than sharks are at smelling blood.
— On geosmin, from We know what causes the scent of spring, but what's it actually for? at The Weather Network via Yahoo.



