The hot water you enjoy flowing forth in the Arima Onsen area is quite unique in that it does not originate from volcanic activity like all other hot springs around Japan.
According to reports by many earth scientists, hot springs referred to as ‘Kinsen’ or ‘Golden Hot Spring’ have a deep brownish-red water that is rich in iron and salinity and are said to contain ingredients of the earth’s mantle. Seawater was taken in when the Philippine Sea plate slipped under the Eurasian plate. The seawater was deposited into the minerals and became steam due to the heat of the mantle. 6 million years later this water is found at the surface, gushing forth from Arima’s hot spring. It is supposed that Arima’s hot spring gushed out on seashores about 300,000 years ago.