Salmon

"Salmon are amazing water creatures. Born in our rivers, they mature in the ocean and return to their birthplace to reproduce. During their time offshore, they may travel more than 2,000 miles across the ocean; some reach the coast of Siberia. As each returns toward it's home estuary, it somehow detects the "fingerprint" scent of the river that it must follow back to it's birthplace. Salmon noses can detect the identifying substances in concentrations as low as one part per 3,000,000,000,000,000,000!" - From Introduction to Water in California - David Carle