Japanese law sets the temperature and depth a spring needs to count as an onsen
In Japan, a hot spring is a legal category. Under the Hot Springs Act, water feeding an onsen must reach at least 25 degrees Celsius, come from 1.5 kilometres or more underground, and carry set amounts of minerals such as sulphur or iron. Japan has roughly 25,000 such sources and about 3,000 bathing establishments using their water.
People have soaked in these springs for at least about 1,300 years. Early records mostly describe emperors and members of the imperial family staying at springs for long stretches, when the waters were considered sacred and healing. That tradition of week-long therapeutic soaking has its own name, toji. Dogo in Ehime, Shirahama in Wakayama and Arima in Hyogo are counted among the oldest.
For centuries onsen were bound up with Buddhism, and temples often rose beside the springs. During the Edo period, woodblock prints known as ukiyo-e helped popularise them with ordinary people, and bathing shifted from worship and healing toward getting clean and socialising. Men and women traditionally bathed together, but separation was imposed at most bathhouses after Japan opened to the West in the Meiji era, and mixed bathing is now banned in public baths, although families can book private baths. Resort towns boomed after the Second World War and again during the bubble economy of the 1980s.
Etiquette is taken seriously. Everyone scrubs and rinses thoroughly at a seated shower station before entering the shared water, and bathers go in completely naked; the towels worn on Japanese television are a modesty device for actors. A small cloth may be carried, but it must never touch the bath water, so many people fold it on their heads. Hair is tied up, and many bathers skip a final shower to leave the minerals on their skin.
Tattoos remain a sticking point. By 2015 around 56 per cent of operators barred tattooed bathers, a rule meant to keep out yakuza gangsters with full-body designs. As foreign visitors multiplied, the national tourism agency urged operators in 2016 to admit tattooed foreign guests under conditions, and some towns, such as Beppu and Kinosaki, welcome tattoos openly.
Source: Onsen
