Sansuiro is a solitary inn nestled at the eastern foot of the Tanzawa mountain range, within the Nanasawa hot spring district. Its source was discovered during the Great Kanto Earthquake of 1923, when a helmet-shaped rock crumbled to reveal a natural spring — hence the name "Kabutoyu" (Helmet Hot Spring). The water is a highly alkaline simple hot spring with a pH of 10.1, delivered as 100% free-flowing source water without any disinfection or circulation systems. With a metasilicic acid content of 69.7 mg, it is celebrated as a "beauty water" known for its excellent skin-smoothing properties. Although the cold mineral spring (17.5°C) is heated for bathing, no chemical treatment is applied. Surrounded by dense woodland, the inn creates the illusion of having stumbled upon a remote mountain retreat despite being within the greater Tokyo metropolitan area. In November, the surrounding Japanese maples blaze with crimson color, offering spectacular autumn views. True to its reputation as "the closest secluded hot spring to the Kanto region," this authentic one-inn hot spring can be reached in just about an hour from central Tokyo.