Photos by Michael Yamashita yamashitaphoto / Living in the Wall. Four thousand years ago in far northwestern China, people first began building and living in cave homes called yaodong. These yaodong, or "house caves," are a distinctive form of earthen dwelling especially common in the Loess Plateau, the area of northern China known for the loose, shifting, sandy silt that is deposited and built up by dry winds blowing across the Gobi desert. The caves are carved out of a hillside or excavated vertically to form a central sunken courtyard with rooms dug out horizontally around it. The earthen walls of the cave are effective insulators that keep the inside of the structure warm in winter and cool in the summer heat. Though the history of yaodongs goes back centuries, they continue to be used as homes, many with running water and electricity. An estimated 40 million people in northern China live in yaodongs. The Great Wall, with its thick, tamped earth walls, has proven to be an ideal fou...