The Origins of Onsen (Hot Springs)
There are two types of onsen: one is a natural spring of geothermally heated underground water and the other is an artificially bored or pumped spring. Both of these (even if they are man-made) are classed as onsen if they conform to the Hot Spring Law. Classifications of onsen by heat source are divided into volcanic onsen with the heat source from the magma under volcanoes and non-volcanic onsen that have no relation to volcanoes. Non-volcanic onsen are further divided into so-called deep level hot water that rises in temperature according to the geothermal gradient (in which temperature rises with depth), and those with unknown heat sources. A special case exists at Tokachigawa Onsen, Hokkaido. This is a moor spring onsen whose heat source comes from the reaction when ancient layers of plant deposits turn into brown coal.
It goes without saying that volcanic onsen are close to volcanoes and they contain the elements of the volcanic gas source. Deep level hot water is mostly found deep underground in open land or basins, is extracted by boring and may contain salt content from sea water sources or organic matter. There are some non-volcanic onsen that have high temperatures that cannot be explained by ordinary geothermic gradient (Arima Onsen, Yunomine Onsen, Matsunoyama Onsen, etc.). There are a number of theories as to the origin of their heat and elements but none of these are more than hypotheses.
Japanese Onsen
As Japan has many volcanoes, there are many volcanic onsen and there are a multitude of legends and myths surrounding onsen resorts. In addition, onsen that were discovered long ago have had the history of their uses recorded in documents over the ages.
The names of Tamatsukuri Onsen, Arima Onsen, Dogo Onsen, Shirahama Onsen and Akiu Onsen are recorded in documents such as Nihon-shoki (the oldest chronicles of Japan), Shoku Nihongi (the second of the six classical Japanese history texts), Manyoshu (Japan’s oldest anthology of poems) and Shuishu (Collection of Gleanings) as being used in miracles of purification or having received visits from the Emperor of Japan. In the Heian Period Engi-Shiki (set of ancient Japanese governmental regulations) shrine names registry, a number of shrines are recorded as onsen shrines that deify onsen gods.
In the Edo Period, onsen became popular even among the populace at large with publications such as the pictorial onsen guide maps with documentation on onsen cures by Ekiken Kaibara, Gonzan Goto and Yoan Udagawa. During this period, baths were divided into crude baths for the general populace and feudal lord or key baths for shogunate officials, prefectural governors and feudal lords and were called “merchant baths” and “samurai baths” respectively. Each clan created a government office for bathing and the offices of bath administrator and bath steward, etc. who collected taxes on bathing.
The custom of seasonal bathing such as New Year bathing, winter hot spring cures, blossom hot spring cures and autumn hot spring cures was common mainly among the general population and people generally travelled to the same onsen resort every year to promote health and relieve fatigue. Further, the “hot spring cure custom” that remains in modern times originated in the Edo Period and has given rise to cures that utilize the characteristics of onsen such as sand baths, waterfall baths, steam baths and time/temperature-regulated baths.
In the Meiji Period, scientific research into onsen became more and more popular and, from the Showa Period onwards, owing to advances in medical hydrology and analytical chemistry, the medical benefits of onsen were proven and onsen users became more widespread. It can be said that it is natural that, historically, traditional Japanese inns (ryokan) for people to stay in opened in areas where onsen are found.