The Tonkararin or Tonkararin tunnel structure is a 464.6 meter long, tunnel-like structure of unknown origin, situated in the Nagomi machi, Kumamoto Prefecture. It lies on a plateau called Seibaru Daichi, around which are many kofuns or tumuli, such as Eda-Funayama Kofun. It consists of very sharrow natural slits of the ground covered with stones ...