As the saying goes, what goes up must come down—and, as it turns out, a lot of what goes up comes down into the world’s oceans.

Iron particles, released by human industrial activities, are one example of a pollutant that goes into the atmosphere and eventually settles into the sea. Now, new research suggests that human-emitted iron is accumulating in the ocean in much greater quantities than scientists previously estimated. And it may also be dissolving into the water more easily than suspected.

The consequences are still unclear, but they’re worth investigating, scientists say. Iron is one of the key nutrients that tiny phytoplankton organisms in the ocean need to thrive…