Musashi-Koyama Onsen Shimizu-yu

Shimizu-yu is a neighborhood public bathhouse (sento) located in Koyama, Shinagawa Ward, Tokyo, a five-minute walk from Musashi-Koyama Station on the Tokyu Meguro Line, just off the bustling Musashi-Koyama shopping street. It was founded in 1924 by Saneki and Utako Kawagoe, and has been run by the same family for three generations since. When the second-generation owner faced a business downturn, he defied the conventional wisdom that no hot springs exist in Shinagawa and gambled on drilling for a hot spring, successfully tapping a natural black-water spring (the first source) at a depth of 200 meters in 1994, a discovery that reportedly tripled the number of customers. Under the third-generation owner, the idea that a bathhouse could just as well have two hot springs led to the drilling of a second, 1,500-meter-deep source in 2007, yielding a therapeutic-grade spring named Ogon-no-Yu (Golden Spring). Having two distinct natural hot springs at a single sento is rare nationwide, and Shimizu-yu is recognized as the first public bathhouse in Japan to be certified as both a deep-source hot spring and a therapeutic spring.

Shimizu-yu offers two natural hot springs of different water quality. The first is Kuro-yu (Black Spring), a sodium bicarbonate spring rich in metasilicic acid, a so-called mohru (humic) spring, drawn from strata more than a million years old. Its source temperature is 18.6 degrees Celsius with a pH of 8.3 (alkaline), and it is rich in humic acid and minerals. The water is a deep amber to dark brown color with the toasty aroma characteristic of mohru springs. Because the source water is cool, it is heated, circulated, and chlorine-treated, with high-concentration nanobubbles added to enhance the bathing experience. The second is Ogon-no-Yu (Golden Spring), drawn from a depth of 1,500 meters and classified as a strong sodium chloride spring (hypertonic, mildly alkaline). Its source temperature is 38.0 degrees Celsius with a pH of 7.8, and its total dissolved solids reach an intense 18.66 g/kg, rich in iodine. The water has a deep milk-tea color with a faint petroleum-like scent, and is supplied on a source-flow (kakenagashi) basis without dilution or added heat.

The bathing area includes an indoor bath (with a hyper jet bath, an electric bath, and a jacuzzi) and a two-tiered open-air bath, with Ogon-no-Yu on the upper level and Kuro-yu on the lower level, plus six deck chairs in the open-air area. The facility also has a dry sauna at around 90 degrees Celsius (an additional 450 yen) with a cold plunge bath, and a women-only ganban-yoku rock-bed sauna (1,500 yen for 45 minutes). There are 100 changing lockers each for men and women, plus rentable lockers available for an annual fee of 2,400 yen. A free rest area is provided on the second floor, and the front desk sells hot-spring eggs, drinks, and light snacks. Business hours are 12:00-24:00 on weekdays, Saturdays, and public holidays, and 8:00-24:00 on Sundays; the bathhouse is closed on Mondays (open if Monday falls on a public holiday). Admission is 550 yen for adults, 500 yen for junior high school students, and 200 yen for elementary school children, with infants free. Parking for 10 cars is available, free for two hours with bathing use.