Phones and computers contain dangerous metals like lead, cadmium and mercury, which can contaminate the air and water when those products are dumped. It's called electronic waste, or e-waste, and the world produces a lot of it: 20 to 50 million tons a year, according to the UN — enough to load a train that would stretch around the world. The U.S. is by far the world's top producer of e-waste, but much of it ends up elsewhere — specifically, in developing nations like China, India and Nigeria, to which rich countries have been shipping garbage for years.