Japanese tunnel_code? [????- ]

Local hikers named it as the Coded Cave of which I have no idea why. It's entrance is small but one can just bend a little bit for entry. Once inside, it's wide enough to fit a taxi. The tunnel looks straight and has a fresh breeze, very comfortable especially in such hot and stuffy summer. Yet, as there seems to be lots of bats poo on the ground, we just ventured for about 20m before turning back, forgetting to sort out why it's so named. Another reason to discourage us was the sound of dripping water from the ceiling on to some kind of puddle.

IMG_20200825_JapaneseTunnel_code.jpg, by H.Lo