A secluded single-inn hot spring nestled at the foot of Mount Sodayama, about 5 kilometers north of central Susaki City in Kochi Prefecture. The spring is said to have been opened during the Heian period, giving it a heritage of approximately 1,200 years. According to local legend, Kobo Daishi (Kukai), the founder of Shingon Buddhism, discovered this naturally welling spring during his pilgrimage around Shikoku and introduced it to the villagers as a healing medicinal water. Formerly known as Kuwadayama Onsen, the inn was renovated in 2012 and reopened in 2013 with new open-air baths under the new name Sennen no Biyu Sondayama Onsen Wa (YAWARAGI). The spring is classified as an alkaline simple sulfur cold mineral spring with a pH of 9.5 and a source temperature of 18.3°C. Its abundant flow rate of 360 liters per minute is among the highest in Shikoku, and the silky, slippery texture of the water has earned it the nickname Bijin no Yu (beauty spring) and W Bihada no Yu (double beauty spring). The mineral-ion-rich water leaves the skin smooth and supple. The intimate inn offers four rooms in the main building and three detached cottages. The men's bath Sonda Yama no Yu and the women's bath Beppin no Yu both feature indoor and open-air baths, all supplied as fresh source flow-through (kakenagashi). Day-use bathing is available year-round from 9:00 a.m. to 9:00 p.m., welcoming both local regulars and travelers.