Ten years into his trek, Salopek is walking along the Yellow River in Shaanxi, China.

Over the past couple of months, Salopek and his walking partners have attempted to follow a network of ancient roadways. First, they walked the old Shu roads of western China, “a great dissolving road system of antiquity.” These old trade routes wind through village waystations and past some of the oldest remnants of the Great Wall of China.

As they meandered through Shaanxi, they also took the Qin Highway, a 450-mile road built more than 2000 years ago and stretching up into Inner Mongolia. Researchers remain unsure why it was built.

Along the entire route, Salopek encounters an aging population, as young people increasingly migrate from rural areas into China’s megacities.