Shionoha Onsen Yamabatoyu is a hidden hot spring inn standing on the shore of Oosako Dam Lake in Kawakami Village, Yoshino District, Nara Prefecture. With a history spanning over 1,000 years since its opening in the Heian period, this legendary spring is said to have been discovered by En no Gyoja, the founder of Shugendo mountain asceticism. During the Edo period, it thrived as a medicinal bathing destination. In 1694, woodblock prints were carved to promote the spring, and these original blocks are preserved to this day. When the Oosako Dam was completed in 1973, the original spring was submerged beneath the reservoir. The Nakamura family subsequently drilled 150 meters underground to tap a new source, reopening the inn in 1977.

The spring water is classified as a sodium-calcium chloride and bicarbonate spring (carbonated bicarbonate spring) with a source temperature of 39.6 degrees Celsius and pH 6.6. An abundant 500 liters per minute gush forth naturally, all provided as free-flowing source water without dilution or recirculation. The water contains an exceptionally high concentration of free carbon dioxide at 768.3 mg/kg, making it one of Japan''s rare natural carbonated springs. Initially colorless and transparent upon emerging, the water undergoes a dramatic transformation to golden-brown as calcium carbonate precipitates upon contact with air. These deposits form travertine (calcium carbonate sinter) that grows at a rate of 1 to 2 centimeters per year, completely encrusting the bathtubs and plumbing.

The facility features a main indoor bath constructed from cedar logs and an outdoor bath carved from a massive zelkova log, offering panoramic views of Oosako Dam Lake and the Oku-Yoshino valley. Years of travertine accumulation have rendered the wooden bathtubs unrecognizable as wood, creating an otherworldly, cave-like appearance that stands as the spring''s most extraordinary attraction.