Within the Akiyoshi-dai Quasi-National Park, Akiyoshi-dō is the largest limestone cave in Japan. It extends about 10km (though public access is limited to a 1km section), at some points 100m wide, and a river flows through it. The watery reflection of the towering cave walls gives the dizzying impression that you’re walking over a deep ravine. There’s a paved route, push-button information points, and an elevator in the middle to a lookout.
Despite the development here, it's a good side trip from Yamaguchi or Hagi. For more on the cave and the surrounding plateau region, a great area for nature walks, go to www.karusuto.com. Information is also available at tourist offices in Yamaguchi and Hagi.