Hidden in the countryside of northern Kyushu is a mysterious set of human-made tunnels. Some are so narrow only a child could crawl through them,while another is more than 13 feet (four meters) tall. Altogether, the tunnels speed for nearly 1460 feet (445 meters) on a mountain slope.
When the network is tunnel was built is unknown, and it's also unclear who carved them and why. Uphill,at the end of the tunnel, is a shrine with elegant wood carvings. Downhill are a few kofun (ancient burial mounds), or which you can visit.
Source: atlasobscura.com