The origin of Arima Onsen was a stratum containing old seawater called serpentinite.
The stratum is carried to the upper mantle by the subduction of the Philippine Sea plate, dehydrated, and is a hot spring directly connected to the plate, which is rare in the world and springs from 60 km underground.
Arima Onsen is known for having the highest salt content in Japan.
The original serpentinite is said to be a stratum 6 million years ago, and it contains old seawater when human beings stood on two legs, and the salinity of Arima Onsen is equal to that. It is a hot spring that is truly timeless.