A large number of Onsen is used as a therapeutic bath from the old days and a hot water purification place at Nanki Kumano. Using the hot spring where Shirahama Onsen, Tsubaki Onsen, Katsuura Onsen and Yukawa Onsen are natural gush, it was developing from the old days. Shirahama Onsen (Muro-no-yu) is ranked as Japan three old hot spring and Japan three large hot spring as hot water from the Asuka period (around 7th century). Yukawa Onsen is one of the spas where history is oldest in the Wakayama prefecture which continues from the end of 5th century. Tsubaki Onsen and Katsuura Onsen are the spa where the history which continues from the Edo Period is here. These Onsen was born in an area along Kareginada arcuate dike and Kozagawa arcuate dike.
These days’ volcano doesn’t exist in Kii Peninsla,but much Onsen is gushing. Therefore it’s contentious from the old days about a source of heat of Onsen, and depends on residual heat of the magma out of which Kozagawa arcuate dike and Kareginada arcuate dike were formed before 14 – 15 million years ago high-temperature fluid, a crack, it was being thought that I rose and gushed. But I rise and the way of thinking by which we assume that It gushed is advocating the crack where high-temperature fluid by dehydration from the plate into which present sinks is an arcuate dike and the neighborhood in recent years. For a warm action, a hydrostatic pressure action, a buoyancy action, a medicine reason action and the change of air effect to have the effect useful for human healthy increase on Onsen, an emperor found its effect, and dropped in in case of a visit, and a seigneur was visiting in recuperation.