Tucked away in the innermost reaches of the Nishi-Tanzawa mountains, upstream of Lake Tanzawa, Ao no Sanso is a small 10-room inn and the only member of the Japan Association of Secluded Hot Spring Inns in Kanagawa Prefecture. The inn channels its self-springing pH 10.5 highly alkaline source water into six baths without any dilution, heating additives, or chemical treatment. It serves as a model facility for the Japan Spa Association's natural hot spring certification program, with all baths meeting the standard. The sprawling 10,000-tsubo grounds attract over 60 species of wild birds year-round, while early summer brings forest green tree frog egg clusters and firefly viewing tours. Inside, a gallery dedicated to Koji Fukiya, a lyrical painter who was evacuated here during World War II, creates a unique fusion of art and nature. Despite being just 90 minutes from central Tokyo by car, the inn offers a genuine secluded hot spring experience enveloped in the profound silence of the deep mountain village.