JAPANESE HOT SPRINGS
Onsens are natural hot springs, born from volcanic activity that is abundant in the country. The practice of immersion in these thermal baths for healing, spirituality and rejuvenation spread throughout Japan along with Buddhism during the 6th century. Some evidence suggests that Buddhist monks were responsible for founding the first spas, or at least something like the modern spas in the country
Because they are based around natural formations some onsens have been used for thousands of years. One such place is Dogo Onsen, located on the island of Shikoku. It is believed that it has been used for at least 3000 years. Mentions to this onsen were found in historical texts, illustrating him as the great leveler, receiving gods, emperors and peasants. There is a certain cultural protocol to follow in a Japanese thermal bath (nudity is mandatory and often men and women share the same space, for example). See here for an etiquette guide before your visit to an onsen.