A huge ancient stone city buried for thousands of years in northern China has just become even more mysterious. Archaeologists working at the Houchengzui Stone City in Inner Mongolia have uncovered a hidden underground tunnel network dating back roughly 4,300 to 4,500 years.
The tunnels were reportedly hidden beneath one of the largest and most heavily defended early stone cities ever found in the region. At first glance, the site already looked impressive enough with massive walls, defensive gates, and layers of protection everywhere. Then researchers discovered something underneath it all.
