Researchers have debated for a long time whether dumping iron into the ocean could ameliorate climate change. Iron encourages the bloom of tiny algae called phytoplankton, which take in carbon dioxide (CO2) dissolved in the ocean for photosynthesis; that process in turn draws atmospheric CO2 into the surface waters. Most scientists remain skeptical of whether iron fertilization will lead to greater carbon sequestration. But a company called Planktos, based in Foster City, Calif., has been forging ahead with such plans. Its latest target: 10,000 square kilometers of the equatorial Pacific, 600 kilometers west of the Galápagos—by far the most ambitious and controversial iron-seeding plan yet.