In the mid-1970s, Japan was experiencing tremendous growth and modernization. The president of the tour company where I worked was interested in promoting small scale, off-the-beaten-path properties, with the purpose of helping people reconnect with their authentic selves. He believed that this was the real meaning of travel - backed up by personal experiences with innkeepers who afforded the time to interact meaningfully with their guests, and who provided an environment where "souls could feel at home".
That president actually coined the word for "secret hot springs" - 秘湯 (hi-tou)
It wasn't until writing this that Glenn and I realized that, without talking about it, we had arrived at much the same conclusion and purpose - it even showed up in an early marketing piece we developed:
There is a book that was published (in Japanese) which lists all of the designated "secret hot springs" - we keep a copy at the inn. There is also a website - 日本秘湯を守る会 公式Webサイト (hitou.or.jp)
One example of the kind of property designated as "secret" is the apparently not-so-secret-anymore Tsuru-no-yu (there are many videos on YouTube), located in Akita Prefecture in northern Japan:
(English website)
Glenn and I visited in the middle of winter - the snow was deep, and we stayed in a 300-year-old building with an open hearth in the room. The "hot pot" cooked in our room was a rustic stew of chicken, Indian yam, and other vegetables. The outdoor bath, cloudy and blue with minerals, was unique. It was the most "Japanese" Glenn had felt to that point. Imagine what life must have been like that many years ago!