You may be an old hand at onsens but have you ever had a cave bath? The Ryokan Yamanoyado Shinmeikan in Kurokawa has carved its baths directly out of the cliffside, the result is a very steamy, dark and womblike experience. There are female, male and a large mixed bath available.
However you may get a shock in the dark if you are not careful, as the day we went the Mixed Onsen Sign was on the male only bath and the mixed bath had no English signage at all. So if in any doubt which bath to enter ask! At this onsen a mere dousing with the onsen water before entering is sufficient, there is no shampoo or soap available or allowed and do remember to bring your own towel. If not you can purchase a tiny one for 200 yen. the changing rooms are compact (code word for tiny!) just leave your clothes in one of the baskets provided, there are also coin lockers available for 100 Yen just outside the onsen.
Shinmeikan is easy to find in the heart of the village on the opposite side of the river, linked by some very charming foot bridges. Just cross over the bridge to the hotel reception and ask inside for the bath. A private onsen is also available to rent for 1,500 yen.