Onsen

Onsen is a Japanese term for hot springs and the bathing facilities around them. The word combines on (温), meaning warm or hot, with sen (泉), meaning spring. In Japan, onsen refer to baths fed by geothermally heated groundwater that emerges at the surface in natural springs. The locations are often linked to volcanic zones or tectonic activity.

Geology and water characteristics: Onsen water is heated underground by geothermal activity and reaches the surface with temperatures ranging from warm to very hot. The mineral content varies by spring and influences color, odor, and feel; common constituents include sulfur, iron, chloride, and silica. Some springs are acidic; others are alkaline. These properties are usually described on signage and by operators; health effects are widely claimed but evidence is limited.

Facilities and types: A typical onsen involves a bathhouse with outdoor baths (rotenburo) and indoor baths. Many onsen towns offer multiple springs in one place. Private onsen rooms are available in some ryokan, enabling private bathing. Public onsen may have gender-segregated baths, though some facilities offer mixed-gender arrangement. Etiquette and rules vary by place, but basic practices are commonly observed.

Etiquette: Visitors are expected to wash thoroughly before entering the bath, and to bathe naked, without swimsuits. Towels should not be left in the water, and conversations are kept quiet. Policies on tattoos and photography vary; some facilities require covering tattoos or restrict photography. Long hair is typically tied back, and some onsen prohibit soaking the head. Always check local rules before entering.

Tourism and regulation: Japan hosts thousands of onsen across the country, with famous destinations such as Beppu, Kusatsu, Noboribetsu, Hakone, and Arima. Onsen are regulated and labeled with the spring’s source and mineral composition. They are visited for relaxation, culture, and scenery, with rules that differ by town and facility.