XI’AN, CHINA—Science News reports that high-resolution muon scans of the 650-year-old ramparts surrounding Xi’an’s historic center have revealed fluctuations in the density of one section of the wall. Located in central China, Xi’an was the capital of the Zhou, Qin, Han, and Tang Dynasties, and the eastern end of the Silk Road. Some sections of these fortress walls are up to 40 feet tall and 60 feet thick. Muons are produced when cosmic rays contact Earth’s atmosphere. As they fall to the ground, muons are scattered or absorbed depending on what sort of material they encounter, and can be tracked with sensors. Nuclear physicist Zhiyi Liu of Lanzhou University said the anomalies detected in Xi’an’s wall could be construction flaws or perhaps hidden structures. Conservationists will now determine how to investigate the potential anomalies, he concluded. To read about 2,000-year-old bronze mirrors unearthed at a cemetery in the Xi'an suburbs, go to "Mirror, Mirror."