What is Onsen? Onsen is the Japanese word for hot spring. It is a term often used to describe the bathing facilities and inns around these hot springs or sources of geothermally heated water which literally `spring up` in areas where volcanic activity is present. Japan, as one of being a very volcanically active country, is covered with them that were scattered along its length and breadth. According to Preservation and Use of Hot Springs, Ministry of the Environment, there are more than 3,000 registered hot spring resorts in Japan and around 30,000 resort spas. They are fascinating places where people from Japan and overseas can all enjoy themselves. Japan's hot spring resorts are ready to welcome everyone (locals and foreigner tourists)