Katsuura Onsen
Katsuura Bay has almost 200 hot springs, with water bubbling to the surface at 40 degrees Celsius. This bath-perfect temperature has made Katsuura a popular onsen town for generations. Ships from Osaka started docking at the port during the Meiji era (1868–1912), bringing crowds of visitors. The shipping line stopped operating in 1938, but the town’s popularity has endured. There are various onsen baths in local resorts, as well as free public footbaths (ashiyu) that overlook the bay. The mineral content of each spring varies slightly, and it is not uncommon to see small flakes of minerals in the water.
The water is heated by geothermal energy rather than volcanic activity. A subterranean formation called the Kozagawa Dike is thought to be the rock pathway that brings the heated water to the surface. The dike was formed 14 million years ago when a vast magma chamber exploded and created a volcanic depression, known as a caldera. The cooled magma now forms a 22-kilometer long dike. Almost all springs in Katsuura bubble to the surface naturally without the use of pumps.