Katsuura Bay

Nearly 200 hot springs surface along Katsuura Bay at a bath-ready 40 degrees Celsius, and the town has been drawing visitors since Osaka ships first docked here in the Meiji era. The water is heated by geothermal energy channeled through the Kozagawa Dike, a 22-kilometer formation created by a volcanic explosion 14 million years ago. Every spring rises naturally, without pumps, and the mineral content varies subtly between sources. Free public footbaths with bay views are open to all, and resort onsen round out your options.