Fukushima Onsen

Fukushima Onsen in Okayama prefecture, Japan, is a hot spring facility with radioactive mineral waters — in the classical Japanese balneological classification, 'radioactive' refers to naturally occurring radon dissolved from radioactive basement rocks, a type of spring water associated in traditional Japanese thermal medicine with circulatory stimulation, metabolic activation, and rheumatic relief in the low-level hormetic tradition of radon therapy practiced at European and Asian spa resorts since the late nineteenth century. Okayama prefecture's geothermal landscape is shaped by the Chugoku Mountains and their granitic basement, which naturally contains low levels of uranium and thorium whose radioactive decay produces radon that dissolves into spring waters percolating through the rock. This onsen — distinguished from the famous Takayu Onsen mountain resort in Fukushima City which shares a similar name — offers the therapeutic radon-bearing mineral waters of Okayama's inland mountains to bathers seeking both relaxation and the mild physiological stimulation historically attributed to naturally radon-enriched thermal springs.