Yumen Onsen, located in Misuminaka, Nagato City, Yamaguchi Prefecture, is an ancient hot spring whose origins are traced back roughly 1,200 years to the famed Buddhist monk Kukai (Kobo Daishi). According to local legend, on his return from Tang China, Kukai received a dream prophecy that led him to discover the spring, giving rise to the name Yumen (literally derived from yume, meaning dream). Another tradition holds that the name comes from a wounded white rabbit that healed itself in the waters, with the kanji 湯免 alluding to the rabbit (men). Long associated with the esoteric Shingon Buddhist faith and Shugendo mountain asceticism, Yumen has endured as a sacred healing spring of the Choshu region. Yumen Kanko Hotel Yume no Sato is the sole inn here, preserving the source and operating both a traditional ryokan and the popular public bathhouse Usagi no Yu, frequented daily by local residents. The source, named Yumen Onsen Distribution Facility Mixed Spring No. 2, 3, 4 and 5, is classified as an alkaline simple hot spring with a pH of 9.06 and a source temperature of 29.4 degrees Celsius. Its radon content reaches 27.27 x 10^-10 Ci/kg, said to be roughly four times the typical level and ranking among the highest radium concentrations in western Japan. The water is colorless, transparent and odorless, with a soft, silky texture renowned for relieving neuralgia, muscle and joint pain, chronic skin conditions, and for its beautifying effects on the skin. The Usagi no Yu public bath operates as a true free-flowing source bath (gensen kakenagashi) and retains the atmosphere of an old healing spa where locals gather day after day. The hotel also offers a private rental bath called Yura, featuring an indoor tatami-floored tub and an outdoor bath, alongside large guest baths for overnight visitors (open 6:30-9:00 and 16:00-23:30). Day-trippers can enjoy the public bath for an exceptionally reasonable 350 yen. Meals showcase the bounty of the Sea of Japan along with Yamaguchi specialties such as fugu blowfish, Mi-Ran wagyu beef, Choshu chicken, and Mutsumi pork, and are also available to day visitors at the restaurant Tsuki no Usagi by reservation. With a 100-mat banquet hall available for groups, the inn balances historical depth with modern hospitality, offering an authentic encounter with one of Nagato's oldest sacred hot springs.