Archaeologists working in China recently revealed a series of extensive underground tunnels believed to be thousands of years old. Researchers working at the ancient stone city of Houchengzui, northern China (Inner Mongolia Autonomous Region), uncovered the extensive network of tunnels thought to be at least 4,300 years old, according to Heritage Daily. The oval-shaped Neolithic city is roughly 341 acres across and home to ancient transportation and defensive networks. Only six underground tunnel passages have been discovered so far, each between 1.5 and 6 meters depth... They’re also around 1 to 2 meters high, and 1.5 meters wide. Extensive tunnel...