Overview
An onsen (温泉, onsen) is a type of hot spring (熱水泉, nessuisen) that gushes out of the ground containing medicinal ingredients. Under Japanese law, a hot spring is defined as having a temperature of 25 degrees Celsius (77 degrees Fahrenheit) or higher, but it can also be defined as having a temperature lower than 25 degrees Celsius, depending on the ingredients contained in it. If the water is neither warm nor has ingredients that do not meet legal standards, it is called a cold spring (冷泉, reisen).
Most Japanese people probably imagine "onsen" as something to be soaked in as a bath. The culture of therapeutic bathing has developed since ancient times, with the expectation that the medicinal properties of hot springs will promote health and relieve fatigue. In Japan, bathing in onsen is also often enjoyed as a leisure activity. There are indoor and outdoor baths, often using cypress and rock baths.
Often, there is a blobby brown or white floating substance called "yunohana" (hot water blooms). This is not dirt, but rather a medicinal ingredient in the hot spring water.
There are approximately 3,000 hot spring resorts in Japan (Hokkaido and Nagano prefectures in particular are outstandingly large), and there are "hot spring towns" throughout the country, which means that Japan is a volcanic country and rich in underground water.
Countries that, like Japan, are blessed with hot springs include Italy, Turkey, Hungary, and Iceland. There are also hot springs scattered around the Korean Peninsula, France, and Germany, where there are no active volcanoes, and hot spring culture has taken root. However, many of them are different from those in Japan, such as drinking hot spring water, using hot spring water as a steam bath (sauna), enjoying swimming in a hot spring pool, and not taking a bath naked.
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