A secluded white-walled, two-story traditional inn standing near Yumigahama Beach in Oyano-machi, Kamiamakusa City, Kumamoto Prefecture, about 15 minutes by car from the gateway of the Amakusa islands. Founded in 1968, the inn is a member of the Japan Secret Hot Spring Preservation Association (Nihon Hito wo Mamoru Kai) and a participating facility of the Kyushu Onsen Pilgrimage. Its most distinctive feature is a 33-meter-long hand-carved cave bath, dug entirely by the family over approximately seven years, where guests can soak in a mystical setting surrounded by rugged rock walls. The inn boasts two distinct natural springs — Shiroyu (White Spring) and Akayu (Red Spring) — with four bathtubs each for men and women, allowing guests to enjoy a varied bath-hopping experience. Shiroyu, drilled in 1977 at 33 degrees Celsius, is an alkaline simple spring with a pH of 8.0, gentle on the skin and ideal for long, leisurely soaking. Akayu, originally tapped in 1996, was damaged in the 2016 Kumamoto Earthquake but successfully revived as Shin-Akayu (New Red Spring) after a 1,000-meter bore and two years of major construction. The new spring emerges at about 50 degrees Celsius with a pH of 6.6 and is classified as a carbon-dioxide-rich sodium chloride and bicarbonate spring, boasting an exceptional bicarbonate content of 4,006 mg/kg — among the highest concentrations of natural carbonated water found anywhere in Japan. Inside the hand-dug cave, a stone bath of Shiroyu, a log-framed bath of Akayu, and an open-air bath are arranged, all delivered as free-flowing spring water that, combined with the otherworldly cave atmosphere, captivates fans of secluded hot springs.