Previous research had shown that fog sampled in and around Santa Cruz, California, contained the heavy metal mercury—the silvery liquid found in old thermometers—but no one knew the source. New results presented at the American Geophysical Union conference in San Francisco today show that the mercury is likely coming from the ocean.

The minute amounts of mercury aren't enough to harm someone walking around in the fog—you could find more mercury in a can of tuna, said atmospheric chemist Peter Weiss-Penzias at the University of California, Santa Cruz. But fog is a big player in the central California coast's water cycle. And since mercury accumulates in plants and animals, getting more concentrated as it moves further up the food chain,