Yifeng Wang holds a piece of banded iron during a visit to an aquarium. Wang and colleagues have proposed an explanation (published in Nature Geoscience) for the precipitation of banded iron deposits in the planet's oceans billions of years ago. A key component of the process, the researchers found in computer simulations, may have been the absence of aluminum in early oceanic rocks. That absence chemically favored the formation of banded iron formations. The continual enrichment of oceanic crust by aluminum as Earth evolved ultimately ended the era of iron band formation.