River passage in Akiyoshido, 1975. Public Domain.

The karst plateau of Akiyoshidai is the largest karst area of Japan covering some 130 km². The plateau is a Quasi National Park (Shuho-cho) named 秋吉台国定公園 (Akiyoshidai Kokutei Kōen), the caves are Natural Monuments. The park is managed by the local prefectural government. It is also a National Geopark, a Ramsar Site and wetland of international importance.

The Paleozoic limestone of this area is a petrified coral reef of Paleozoic age. Reef limestone was uplifted during the Akiyoshidai orogenic movement. It contains over 400 limestone caves, including Japan’s longest cave 秋芳洞 (Akiyoshidō).