Millions of tons of plastic slip into oceans every year. Some floats and travels slowly with the currents, endangering the health of marine animals. The rest is hardly visible but is far more dangerous. Tiny bits of plastic sprinkle the ocean's surface or mix into the sandy seafloor and beaches. It ends up inside birds, fish, and other animals, harming them-and ultimately humans. Experts struggle with fear and hope as they work to stop the flood of plastic threatening living organisms across the globe.