Shinzaemon no Yu is a hot spring facility in the Zao Onsen area of Yamagata City. It is particularly popular for its large outdoor baths (rotenburo) with their milky blue water. These baths are said to feature one of the most acidic hot spring waters in Japan which is believed to bestow certain health benefits on anyone who baths in it, though people with sensitive skin should be careful. If your skin is sensitive, you can use other baths within the facility that mix the natural hot spring water with ordinary water to decrease the acidity. As with most onsen in Japan today, the baths for men and women are separated, but there is a restaurant and souvenir area that you can enjoy together if you are visiting with someone from the opposite sex.