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Great Green Wall. Workers planting pine (Pinus sp.) trees in the Kubuqi desert, Inner Mongolia, China, as part of the Great Green Wall project. This project involves planting a strip of trees and vegetation to hold back the desert sands and prevent them encroaching on Beijing. (KEYSTONE/SCIENCE PHOTO LIBRARY/THIERRY BERROD, MONA LISA PRODUCTION/ SCIENCE PHOTO LIBRARY)