Shiriyaki Onsen is located in Nakanojō Town, Agatsuma District, Gunma Prefecture, and is an extremely rare type of hot spring that wells up directly from the riverbed of the Nagasasa-zawa River. The name “Shiriyaki” (“butt-burn”) comes from the old practice of sitting on stones heated by the spring water to treat hemorrhoids. It’s a calcium-sodium sulfate-chloride spring, and between the weirs built across the river a vast open-air bath—known as “Kawa-no-yu”—is formed. The water temperature hovers around 38 °C but can vary with weather conditions. It’s free and open 24 hours a day. There’s no attendant at the river bath itself; management is handled by Nakanojō Town. While the bath is technically mixed, wearing swimwear is recommended.