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 (4 meters) tall. Altogether, the tunnels run for nearly 1,460 feet (445 meters) on a mountain slope.

The tunnels are surrounded by mysteries. It’s unclear when this network was built, who carved them, and what for. Not even the origin of the name is known, seemingly an onomatopoeia of a stone thrown into a tunnel, though it may be of Korean origin.

Numerous theories have been proposed since the 1970s, and all of them remain mere theories to this day. The stonework is done in a similar way to the Egyptian pyramids, which is very rare in ancient Japanese structures, but what it means is another mystery.

Uphill, at the end of the tunnel, is a shrine with beautiful wood carvings. Downhill are a few kofun (ancient burial mounds), which you can visit.