Oku-Yagen Onsen Ganso Kappa no Yu is a free rustic open-air bath located in the mountains of Yagen Valley on the Shimokita Peninsula in Mutsu City, Aomori Prefecture. Located about two kilometers up the mountain from Yagen Onsen, two open-air baths called Ganso Kappa-no-Yu and Fufu Kappa-no-Yu are about a five-minute walk from one another. The name originates from a kappa (water sprite) legend. About 1000 years ago, Jikaku Daishi, a wandering priest who founded Mount Osorezan, fell from a cliff and was injured after losing his way. According to legend, he was miraculously healed by a kappa who put him in the hot spring water. When the priest woke up, he found himself wrapped in a large butterbur leaf and placed in an outdoor bath. Impressed that his pain had completely disappeared, he named it Kappa no Yu. The opening of Yagen Onsen dates back to 1615 (Genna 1st year) when Jouchi Kurataro, a defeated warrior from the Toyotomi side in the Summer Siege of Osaka, fled to this area and discovered the hot spring, making it over 400 years old. The bathtub is built along a mountain stream and features mysterious emerald-green waters. The source temperature is 59°C, the bath temperature is 46°C, and the water quality is weakly alkaline simple hot spring, said to alleviate conditions such as gastrointestinal diseases, skin diseases, gynecological diseases, arthritis, rheumatism, and fatigue. A basic changing room and dial-lock valuables lockers are provided. Previously a mixed bathing facility, it was temporarily closed by the city in March 2010 due to concerns, and reopened in April 2011 with separate bathing times for men and women. Currently, men's bathing hours are 7:00-9:00, 11:10-13:00, and 15:10-17:00, while women's hours are 9:10-11:00 and 13:10-15:00. Admission is free, and parking for 10 cars is available. To protect the natural environment, visitors should refrain from using soap or shampoo. The operating period is from April 1 to November 10, closed in winter, and closed on Wednesdays.