This otherworldly limestone cavern, Japan's largest, lies roughly halfway between Hagi and Yamaguchi. Although the cavern is roughly 6 miles long, only a bit less than a mile is open to the public. The path is easily accessible and lighted just enough for you to marvel at the size, but dim enough to retain a sense of wonder and mystery. Although droves of tour groups can ruin the atmosphere on weekend mornings, they have mostly cleared out by the afternoon.
The Akiyoshi Plain above the cave is a beautiful limestone karst, and makes for a pleasent spring or autumn hike. The observatory (accessible by elevator from the cave) offers impressive views in every season. If you plan to cross from the San-in to the San-yo region, stopping a couple hours at Akiyoshi-do is highly recommended. Bocho Bus service run twice daily from the Higashi Hagi Station (10:15 and 1:15, ¥1,760) stopping at the Hagi Bus Center on the way; the ride takes roughly an hour. Nine buses a day make the 40-minute, ¥1,140 trip to and from Shin Yamaguchi station, between 6:45 am and 6:40 pm.
Hirotani Shuhocho Akiyoshi, Mine, Yamaguchi-ken, 754-0511, Japan