Akiyoshi Do in central Yamaguchi Prefecture is the largest cavern in Japan and one of the largest in East Asia. Located under Akiyoshidai, the largest karst (limestone plateau) in Japan which is home to about 200 caves.
It is about 10 kilometers in length, but only the first kilometer is open to the public.
It truly is cavernous inside, reaching 100 meters at its widest point and 80 meters at the highest point.
It took about 300,000 years for the water to erode the cave system. There are numerous "features" with evocative names but that will be in a later post. If you dont want to walk all the way back out there are two other exits, one an elevator up to the plateau.
Akiyoshido is located in Mine City and is open 365 days a year.