Onsen, or natural mineral rich hot springs, have been incorporated into the Japanese culture for thousands of years.  Japanese and foreign tourists alike turn to Onsen for the relaxation and healing qualities that their waters offer.  Though onsen were traditionally located outdoors, they can be either enclosed or exposed to the elements, humble or lavish, ancient or modern.  What distinguishes a pool of water as an onsen rather than a sento (or indoor public bath houses) is the natural geothermic heating of the water and the chemical components of that water.  It is the chemical composition of onsen water that supports a general belief in their healing powers. 

Often times, onsen are found within the larger context of a Ryokan, which is a traditional Japanese inn.  Historically, onsen towns were formed around areas of volcanic activity where the waters were rich with minerals and warmed by the earth.  These towns, in turn, attracted visitors from larger cities and non-onsen villages.  In response to these tourists of the waters, Ryokan inns were constructed.  Today, with both Ryokan and onsen still as popular as ever in the Japanese culture, many artificial onsen are engineered to accompany inns that are not on sites that are geographically compatible to naturally heated hot springs.  Though these pools of water are still referred to as onsen, this is not an accurate labeling, as they are removed from the true intersection that onsen represent between healing, nature, and relaxation.