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Onsen water is an underground resource shaped by water sources, heat, flow paths and dissolved constituents. Japanese classification uses source temperature or specified composition. Water at the source and water treated for a bath describe different stages.
Japan’s 1948 Hot Springs Act recognizes a spring through a temperature of at least 25°C when collected at the source, or through at least one listed substance reaching its specified concentration. Cooler mineral water can therefore qualify; warm water need not be mineral-rich. The legal category also covers certain steam and gases, while this article focuses on water.
Rain and snow can infiltrate the ground, become heated and acquire dissolved constituents before emerging through fractures, faults or drilled wells. The source of the water, the heat and the flow path all matter. Spring water follows those pathways; magma is not piped into the bath.
In volcanic areas, a magma reservoir transfers heat to surrounding groundwater. In the source diagram, pale arrows show infiltrating water, blue arrows show spring water and cyan lines mark faults. The red reservoir below supplies heat while water moves through the surrounding geological structure.
Temperature generally increases with depth, allowing deep groundwater to be warmed away from volcanoes. Sedimentary basins in the southern Kantō Plain contain deep thermal-water resources used by some Tokyo springs. The red arrows in this diagram represent heat, not red water.
Some nonvolcanic springs draw on ancient seawater trapped underground through geological change and warmed by heat from below. This fossil-water origin differs from groundwater supplied by infiltrating rain and snow. It is one possible source, rather than the origin of every nonvolcanic spring.
Temperature, pH and dissolved constituents describe different properties and support different classifications. Waters sharing the legal name onsen can vary across these measures. The 25°C threshold describes a source criterion, not a required bath temperature; colour alone cannot substitute for composition analysis.
Facilities may add water, heat it, recirculate and filter it, or disinfect it. A 2005 Ministry of the Environment notice added disclosure of such treatments and their reasons from 24 May. Reading source analysis alongside treatment information distinguishes water collected underground from water as used in a facility.
Springs may emerge naturally, flow from drilled wells or be pumped. New drilling expanded with postwar economic growth. Development increased ways to use thermal water while making extraction levels and interactions between sources matters for management.
The ministry’s 2026 account identifies development and excessive pumping as possible causes of declining flow and temperature at some sources. It also describes permission systems for drilling and powered pumps. Geological origin explains the resource; management concerns its continuing use.
No. A listed constituent reaching its threshold provides another route.
No. Nonvolcanic deep groundwater and fossil water also supply springs.
Heat, rather than red liquid.
No. It is one source-classification criterion.
No. They describe different stages.
New development and excessive pumping may affect flow and temperature at some sources.