Oku-Iki Sennenyu Hirayama Ryokan stands in the Yunomoto Onsen district on the northwest coast of Iki Island, Nagasaki Prefecture, and is a member of the Association to Preserve Japan's Secret Hot Springs. Legend holds that Empress Jingu discovered the spring in the 5th century and used it to bathe the newborn Emperor Ojin, giving this onsen a heritage spanning more than 1,700 years.

The sodium chloride spring rises at 66.5 degrees Celsius with a pH of 6.6, delivered as an unaltered free-flowing source bath with no dilution or heating. The water emerges clear but turns a striking reddish-brown as iron in the mineral content oxidizes on contact with air, producing the celebrated nigori-yu cloudy bath. Even a brief soak warms the body to the core.

The inn also earns acclaim for its creative local cuisine featuring fresh seafood from Iki waters and pesticide-free vegetables grown on-site. With only eight guest rooms, Hirayama Ryokan offers an intimate retreat immersed in island tranquility and richly colored mineral waters.