Ganban-yoku, or bedrock bathing, is a type of heat bath that was originally imported to Japan from Thailand. It is also called "bathing without hot water"!
Ganban-yoku stones are usually made of granite or silica, and embedded in the floor of a special room and heated to a temperature of around 40℃. The room is air conditioned to keep the overall temperature slightly lower than that of the stone beds.
The heated stone slabs emit far-infrared rays and negative ions. The rays penetrate the human body, increasing our basal metabolism, improving blood circulation, and detoxifying the body by helping eliminating wastes. We can get much more perspiration than sauna!
Do we have to be naked when we use ganban-yoku?
No! We wear special clothes like pajamas, spread towels on the stone, and lie down on it. It's very comfortable and soothing!