Koyahara Onsen Kumagai Ryokan is a secluded single-inn hot spring nestled at the northwestern foot of Mount Sanbe in Koyahara, Sanbecho, Oda City, Shimane Prefecture. The spring is said to have first emerged during the Genbun era (1736-1740), giving it a history of over 280 years. Known for its carbon dioxide-rich waters derived from the volcanic activity of Mount Sanbe, the inn operates under the unpretentious motto: ''We have nothing but the hot spring.'' With only five guest rooms, this small-scale ryokan sits in serene silence surrounded by the deep greenery of Mount Sanbe.

The spring quality is classified as carbon dioxide-sodium/magnesium-chloride/bicarbonate spring, with a source temperature of 37.8 degrees Celsius and a neutral pH of 6.0. The flow rate is 30.8 liters per minute. The source water wells up directly beneath the building and is delivered completely unheated in a free-flowing kakenagashi style, with absolutely no water addition, heating, circulation, or disinfection. The defining feature is the exceptional carbonated water that covers the entire body in fine bubbles, rivaling the famous Shichirida Onsen in Oita Prefecture for its effervescence. The water has a faint brownish tint with a metallic scent, and when tasted, it reveals a sweet flavor with an umami-like quality.

The inn has four private indoor baths, each with subtly different water characteristics and temperatures. Baths are reserved in 50-minute slots, allowing guests to enjoy the waters in complete privacy. The bathtubs and floors are coated with mineral deposits accumulated over many years, creating a unique stalactite-like landscape. Day-trip bathing costs 600 yen for adults and 300 yen for children, but as overnight guests take priority, advance confirmation by phone is required. Overnight stays with two meals start from around 9,000 yen per person.