Japan is dotted with mineral-rich natural “onsen” hot springs, both indoors and outside, many offering a warming dip amid a frozen setting.

Following are questions and answers about the historical, geological and social aspects of hot springs.

What is an onsen?

Although the term onsen literally means hot spring, it can also be used for a cold spring if it contains any of 19 different minerals specified by the onsen law that was enacted in 1948. The law requires spring water to be 25 degrees or above and to contain certain levels of hydrogen ion, fluorine ion, sulfur or any other of its listed components.

Many people soak in hot springs for relaxation and to cure certain ailments.

Why does Japan have so many hot springs?

Volcanoes play a key role in their formation. Japan, 75 percent mountainous and with many of its peaks volcanic, thus boasts more than 3,000 hot springs. There are plenty of volcanoes elsewhere in the Pacific Rim, and hence hot springs, including in the United States, China, Taiwan, South Korea, New Zealand and Indonesia.

In Japan, most hot springs are clustered in volcanic belts in Kyushu and the Tohoku and Chubu regions, activated by four plates across the nation and its surrounding seabeds. Many hot springs come from heated groundwater near volcanoes.

But there are some that are not directly the result of volcanoes. Some hot springs draw their heat from diastrophism or radioactive elements underground, according to a book written in 1994 by Haruo Shirouzu, a professor emeritus at Kyushu University.

There are meanwhile many hot springs in central Tokyo, and these do not originate from volcanoes. Experts say there is a layer of warm water, heated by the Earth’s core, under southern Kanto. Machines are used to bore for this water so it can be pumped into hot springs baths.

The mineral content of hot springs is linked to geographical and geological features. Some studies, for example, indicate a spring’s given proximity to magma below a volcano can determine its mineral makeup.

What ailments are hot springs effective in alleviating?

Hot springs water in general is believed to be good ways to lessen neuralgia, myalgia, rheumatism and dermatosis. Also, the minerals, including carbonic acid, hydrogen sulfide and radium, are believed to be effective against high blood pressure and arteriosclerosis.

Some hot spring resorts are famous for medical treatment. One example is Kusatsu Onsen in Gunma Prefecture. Its springs’ minerals includes sulfide and aluminum.

Hot springs in Europe, including in Germany, France and the Czech Republic, often find more visitors less bent on relaxing and more inclined to seek out the springs’ medicinal benefits. The water is even bottled for drinking.

How long have hot springs been around in Japan?

The country’s three oldest hot springs are believed to be the Dogo Onsen in Ehime Prefecture, Arima Onsen in Hyogo Prefecture and Shirahama Onsen in Wakayama Prefecture, because ancient literature describes emperors’ trips to these places.

Some experts even say Dogo is 3,000 years old because pot chards from that time have been discovered in the area.

Before Western medical science spread in Japan in the Meiji Era (1868-1912), hot springs played more of a curative role. Buddhist monks played an important role in propagating their medicinal merits across the nation. During the wars in the 15th to 16th centuries, wounded warriors sought out the curative effects of hot springs.

Some mythology also links hot springs with physical treatment. Legend has it an injured white heron found Dogo Onsen and flew to the springs every day until its ailing leg was completely cured.

Buddhism and hot springs also share mythological links. Kukai, the founder of the Shingon sect in the ninth century, is said to have discovered many hot springs across the nation, including the Shuzenji Onsen in Shizuoka Prefecture.

Are hot springs resorts now more for refreshment?

Yes. According to a survey conducted in 2005 by Internet service provider goo, 46 percent of 30,456 pollees said they enjoyed “short trips to onsen.” The percentage is higher than those inclined to travel overseas or go to concerts or movies.

But not all hot springs resorts have offered a relaxing, scandal-free soak.

In 1999, a resort in Otaru, Hokkaido, barred U.S.-born Japanese Debito Arudo from entry simply because he looked foreign. In 2005, the Supreme Court ordered the resort to pay him ¥1 million in damages.

In 2003, a hot spring resort hotel in Aso, Kumamoto Prefecture, canceled reservations by a group of Hansen’s disease patients. After the discrimination came to light, the inn was forced to close.

Hot springs facilities have also suffered hygiene problems.

Legionella bacteria, which causes pneumonia or fever mostly in children and the elderly through water particles, is sometimes found in recirculated water.

The worst case was a Legionnaire’s disease outbreak at a spa in Hyuga, Miyazaki Prefecture, in 2002. It led to seven deaths and possibly infected about 300 people.

After the Miyazaki case, the health ministry required such facilities and public baths to check their water quality once a year.

In 2004, it was revealed that a hot spring facility in Nagano Prefecture was adding mineral extracts to its regular water to pass it off as a natural hot springs.

This led to later revelations that several other onsen facilities had been using tap water instead of natural spring water.

Do people bathe naked in hot springs?

It depends on the location. Some new onsen facilities and mixed baths allow or may require users to wear swimsuits. Many traditional onsen, however, are for naked bathing only.

Many onsen are segregated by gender. For young women who tend to shun mixed bathing or people who prefer bathing with their partners or families, there are an increasing number of private onsen that can be reserved and even small private baths in inns, some attached to rooms.

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