Hot springs are a central part of Japanese culture. Hot springs, or ‘onsen’ in Japanese, are one of the best things to visit in Japan, where they’re a massive tourist draw. Onsen are usually in rural areas, where they serve as a hotel, resort and spa. The area where Iwate, Miyagi and Yamagata meet is full of many different onsen, some dating back hundreds of years. Tohoku is home to dozens of onsen, with several whole town given over to hot springs.
In Japan, onsen are usually gender separated, with areas for men and women to soak in hot tubs. There are often many different types of pools, from superheated soaking tubs, to ice cold pools and even a tub that zaps the bather with a small electrical charge that is supposed to cure muscle pain and other illnesses. Some onsen, usually the more remote resorts, have private baths for people staying at the hotels. Others have clothed onsen for the more bashful.
Different towns have different spring sources; Naruko, for example, has very sulfuric water. One caveat about the Japanese onsen is that tattoos are generally considered taboo, so be aware that body art is not always welcome in a Japanese bath. Tohoku is especially rich in onsen and hold a big place in both onsens culture and history. In Japan, even the monkeys love to sit in a hot spring and relax.
So make sure you get your feet wet when you come visit.