Since Japan is a volcanic island, there are hot springs all over the country from north to south. There are roughly 150 hot springs with 14000 individual springs. These natural treasures have been used by many not only for medical treatment but also as a place to communicate with others and relax. Many cultural elements have sprang out just as the hot water from these springs. In recent years there springs have become a part of modern medical rehabilitation and recuperation.

Hot spring bathing is first known to have become popular during the Heian Period more than 1000 years ago. Limited mainly to the aristocrats in the beginning, it spread to the warlords during the Warring States Period (1477-1568), when it was discovered how effective the springs were in healing battle wounds. With the establishment of peace and stability during the Edo period (1603-1867), the custom of bathing in hot springs was adopted by the hard working farmers. It was the most pleasant and economical way they could relax and recover from the rigors of their lives.

Onsen had a set back in the form of postwar American Puritanism, whereas many mixed bathing facilities were prohibited. They are still around today, especially in Tohoku (Northern Honshu), but the damage was done, and most onsen you visit will be separated by sex. Hot spring bathing remains an extremely popular custom for people of all walks of life, and the restorative effects of the springs ensure that this delightful custom will continue.

The Japanese have made an art out of bathing. There may not be more of a quintessential Japanese experience than sitting outside under the stares with a group of strangers soaking in an onsen (hot spring).

Hot spring bathing first became popular in Japan more than 1,000 years ago. It was originally limited to aristocrats, but spread to the warlords when it was discovered how effective the springs were in healing battle wounds. With the establishment of peace and stability during the Edo Period, the custom of bathing in hot springs was adopted by the hard working farmers. It was the most pleasant and economical way they could relax and recover from the rigors of their lives.

Japan’s misfortune of sitting atop one of the most thermally active areas of the world does come with an upside: thousands of natural hot springs scattered across the country. Some of the best hot spring resorts are in areas of great natural beauty, thus making a great travel destination.

Onsen come in all shapes and sizes. Some are more like health spas, and can provide a variety of wellness activities in addition to their healing waters. Others are simple, no-frills establishments that cater to locals who stop by after work for a scrub and a soak. With a little research you’ll be sure to find an onsen resort near your city, and your travel agent may even have package trips available that include transportation if you are looking for an overnight stay. When traveling the countryside, just look for the red onsen symbol. They are a godsend for campers and the perfect way to end a day of hiking or skiing.

There are many different kinds of hot springs for different uses and different features.

Simple Springs	Water temperature is higher than 25c. Has less than 1gram of radical carbon, salt and other mineral elements. Good for neuralgia and rheumatism, also it is mild making it perfect for long term rehabilitations.
Carbonate Springs	Has less than 1 gram of radical carbon and other mineral elements in each liter. Basically close to the Simple Spring water but as you can guess from the name it is carbonated. Many of them are low temperature and cold water springs. Good for heart disease, blood circulation disorders, neurological disorders, female disorders.
Heavy Carbon Soil Springs	More than 1 gram/litter of radical carbon and other mineral elements. Good for rheumatism, neuralgia, chronic skin disorder. Also good for drinking to treat digestive problems and inflammation of the bladder.
Salt Springs	Next to Simple springs in numbers, has more than 1 gram/liter of mineral elements. If the water contains more than 1gram but less than 5 gram of salt it is a weak saltine spring if less than 10 gram then it is a Salt Spring , if more than 10 gram the it is a Strong Salt Spring. Since is highly sufficient in keeping temperature it is called "Atsu no yu" (Warm hot water)
Saltine Sodium Hydrogen Carbonate Spring	Salt springs with Sodium hydrogen carbonate content, alkaline base
Mirabilite Springs	Good for arterioscleroses or cholecystitis and rheumatism. Sulfite natrium base bitter spring. Not recommended for drinking.
Mirabilite Sodium Chloride Springs	Mirabilite springs with sodium chloride content. Good for same usage as Salt Springs and Mirabilite springs.
Gypsum Springs	Good for rheumatism or wounds also help the metabolism when drank. High in calcium content bitter spring.
True Bitter Springs	Sulfite magnesium is the main mineral content , good for rheumatism. Low in numbers.
Iron Springs	Two different kinds of springs: Carbonate iron springs and green vitriol springs. When the water is boiled it turns red. The green vitriol springs contains sulfite iron and strongly acidic vitriol.
Acidic Springs	Indigenous to Japan. Mainly in volcanically active area, high concentration of hydrogen ions. Hard to bath in and should be advised before taking a bath.
Alum Springs	Main mineral element is sulfite alum. Good for skin disorders. Mostly acidic alum or acidic alum green vitriol springs.
Sulfur Springs	Contains Sulfhydryl ions or Sulfur hydrogen, smells like rotting eggs. Good for drinking and treating many disorders.
Radium Hot springs	Contains a certain amount of radium or thoron, good for digestive disorders or rheumatism.

Onsen water is believed to have healing powers derived from its mineral content. A particular onsen may feature several different baths, each with water with a different mineral composition. The outdoor bath tubs are most often made from Japanese cypress, marble or granite, while indoor tubs may be made with tile, acrylic or stainless steel.

Traditionally, onsen were located outdoors, although a large number of inns have now built indoor bathing facilities as well. Onsen by definition use naturally hot water from geothermal springs. Onsen should be differentiated from sento, indoor public bath houses in the city where the baths are filled with heated tap water. Major onsen resort hotels often feature a wide variety of themed spa baths and artificial waterfalls in the bathing area (utaseyu).

Many bathers come for only an hour or so to soak in the waters. Food also plays an important part in the attraction of a particular inn. While other services like massages may be offered, the main reason most people visit the onsen is to enjoy the baths.

Traditionally, men and women bathed together at the onsen, as they did at the sento, but single-sex bathing has steadily become the established custom since the opening of Japan to the West during the Meiji period. Mixed-sex bathing persists at some onsen in the rural areas of Japan, which usually also provide the option of separate "Women-only" baths or different hours for the two sexes, although young children of either sex may be seen in both the men's and the women's baths.

People often travel to onsen with work colleagues, as the relaxed and open atmosphere helps to break down some of the hierarchical stiffness inherent in Japanese work life. However, most visitors to onsen are not work groups but friends, couples and families.

And those famous hot springs don't just attract the Japanese and tourists....

Happy Relaxation!

Footnotes & Links:
-Wikipedia article on Onsens - http://en.wikipedia.org/wiki/Onsen