Sunayu is a large open-air hot spring bath naturally gushing from the riverbed of the Asahi River, directly below the Yubara Dam in Maniwa City, Okayama Prefecture. It has been ranked as the 'West Yokozuna' (Grand Champion of the West) in the national open-air bath rankings. The history of the spring dates back to the Heian period, when the renowned monk Shoku Shonin (910-1007) of Shosha-zan Engyoji Temple in Harima was said to have been guided to these healing waters by a divine child in a dream during a serious illness. After recovering here, the spring became widely known for its medicinal properties. Some theories suggest the spring was used as a therapeutic bath by iron smelters as early as the middle Yayoi period.

The water quality is alkaline simple hot spring (low-tension, alkaline, high-temperature spring) with a pH of approximately 9.0. The source temperature is about 45 degrees Celsius with a flow rate of approximately 60 liters per minute. The spring gets its name 'Sunayu' (sand bath) from the way hot water gushes up through the riverbed sand. As a foot-gushing natural source spring, bathers can enjoy completely fresh source water that has never been exposed to air. The water is rich in metasilicic acid, making it renowned for its skin-beautifying effects.

The bathing area consists of three pools of different temperatures named 'Bijin no Yu' (Beauty Bath), 'Kodakara no Yu' (Fertility Bath), and 'Choju no Yu' (Longevity Bath). The bath is mixed-gender and open 24 hours a day, free of charge. Separate simple changing rooms are provided for men and women, and rental bathing garments are available for women (2,000 yen for day visitors, 500 yen for hotel guests). Towel wrapping is also permitted. Sunayu is one of only 50-60 foot-gushing hot springs in all of Japan, making it exceptionally rare and valuable.