Naturally created hot baths. People usually bathe in them nude or wearing a towel, they are popular in Japan.
Hot Springs are naturally springs of water that are heated through geothermal vents in the ground. They can be found all over the world with temperatures varying from 97 °F to over 99 °F. The island nation of Japan has many volcanic vents since it runs along the segment circling the Pacific Ocean called The Ring of Fire. So named due to the amount of volcanoes that ring the area.
The warm temperatures of the hot spring allows the water to hold and distribute a variety of minerals. Hot Springs are often considered popular vacation spots due to the folklore of their medicinal properties. They can be found with spas and certain clinics.
Hot Springs, like swimming pools and beaches, are often used as an excuse for a series to have fanservice scenes. A hot spring scene is usually accompanied by plenty of convenient censorship or modesty towels.