The waters of Itoigawa Onsen gush forth from a depth of 1,000m, from the huge Fossa Magna fault line that runs the length of Japan from Itoigawa to Shizuoka. The hot water at the source is 100 degrees Celsius and 1,500 litres gush out every minute of every day. The secret of this high-temperature, mineral-rich spring water is the fossil sea water from 15 million years ago that wells up from the Fossa Magna fault. As this water was once seawater it is still slightly saline if you taste it and it is this saltiness that helps the body retain heat once you have sat and warmed yourself in the spring waters. This spring is known for its healing properties for people with sensitivity to the cold and bad circulation.

● Day visit facilities: Hisui-no-yu (10:00-22:00; last entry at 21:30) 

● Accommodation facilities: Hotel Kunitomi Annex (also open for day visits from 15:00-21:00; last entry at 20:00)