At an abandoned hat factory in Danbury, Conn, scientists are testing genetically engineered trees to see if they can be used to remove toxic mercury from the ground. In a laboratory in Raleigh, N.C., another group is working to modify trees to make paper production less polluting and more energy efficient. At Purdue University in West Lafayette, Ind., still more scientists are working on ways to engineer trees so they can store more carbon in their roots as a way of fighting global warming.