If I’m correct, the old Yona tunnel is closed off by large metal door. Upon coming closer to the door, I could feel a light breeze and hear electricity on the other side. I did some research and mostly found Japanese sources, but one source stated that the tunnel was bought out and converted to an alcohol storage facility. The text was translated from Japanese to English, so maybe something got lost in translation. The link I provided below will bring you to a video I made and visually describes what I’m talking about.
by _Jersey_Kid_