Hanesawa Onsen Katoya Ryokan

Katoya Ryokan is the original source inn of Hanesawa Onsen, tucked into the hills of Sakegawa Village in northern Yamagata Prefecture. The spring was born by accident in 1919, when Nippon Oil drilled some 600 metres underground in search of petroleum and struck hot water that surged up together with natural gas. Katoya itself was founded in 1913 as lodging for the drilling crews, making it the oldest inn in the hamlet. Its founder secured the water rights and stabilised the flow, and to this day three inns and one public bathhouse share the single source.

The water is a sulphur-bearing sodium chloride and bicarbonate spring, emerging at 45.2 degrees Celsius with a pH of 8.3 and a total mineral content of roughly 3,697 mg/kg. It is clear and colourless but remarkably slippery, almost lotion-like on the skin, which has earned it a reputation as one of Yamagata's finest bi-jin no yu, or beauty waters. Because the source was drilled for oil, a faint petroleum note mingles with the sulphur aroma. The bath is free-flowing with no added water, so fresh spring water constantly spills over the rim; the posted notice lists year-round disinfection and heating during the coldest months.

The bathhouse is a period piece: a half-octagonal room divided down the middle by a central pillar and a single wall, with the men's and women's tubs meeting at that partition. There is no open-air bath and no sauna, just one indoor bath for each sex in the spare style of a traditional cure inn. Day bathing runs from 8:00 to 20:00 with last entry at 19:30, costing 500 yen for adults and 200 yen for children. The inn has 15 rooms, with two-meal stays starting in the 6,000 yen range.