Snaking hundreds of miles across mountains and plains, the Great Wall of China is an iconic and unparalleled historic site. Construction on the vast network of walls, fortresses and other fortifications began in the seventh century B.C. and continued until the 17th century A.D. The monumental structure has stood the test of time — in part because of a “living skin,” scientists recently found. Researchers wanted to figure out the best way to help protect the Great Wall of China from wind and erosion, according to a study published Dec. 8 in the journal Science Advances. They noticed that the st…