Located along the Koayu River at the eastern edge of the Tanzawa mountain foothills, Iiyama Onsen is a historic spa town recorded in the "Kokoku Chishi" (Imperial Geography) published in 1875. Motoyu Ryokan, true to its name meaning "original source inn," has preserved the foundational spring of Iiyama Onsen. Its most remarkable feature is a pH of 11.3 — one of the highest alkaline levels of any hot spring in Japan. The water softens the skin's keratin layer, leaving bathers with a silky smooth sensation that has earned it nationwide fame as a "beauty water." The inn's distinctive architecture connects various buildings through alleyways reminiscent of a castle town. The cuisine showcases the bounty of the Tanzawa mountains, featuring seasonal specialties such as natural sweetfish in summer, wild boar hot pot in winter, and Atsugi's signature marinated pork. Just 20 minutes from Ebina Service Area and only 60 minutes from central Tokyo, the inn's tranquil setting surrounded by mountains and rivers provides a complete escape from urban life.