Though the text below the image is in Japanese, this link will take you to a report written in English about Tarutama Onsen in the Aso area. The English page doesn’t explain how to get there but there’s a link to a Japanese web page that has a Google map showing the location.
http://kumanago.jp/en/blog/2015/01/01/great-onsens-of-tarutama.html
