Akiyoshi Cave is one of Japan's great show caves, with a public route through a vast limestone system beneath the Akiyoshidai karst plateau. The cave is several kilometres long, and the open section gives visitors a well-built walkway past broad chambers, underground water, and large formations.
The entrance is low in the valley and the route can be combined with the plateau above by elevator or bus. It is an easy half-day from Yamaguchi if you have transport, and one of the strongest natural sights in the region.