You're probably starting to get the sense that the Japanese are generally very reserved, so the sight of hundreds of nude or nearly-nude Japanese people bathing together might initially shock you. If you visit one of the thousands of onsens (hot springs) across the country, however, it's something you will quickly become accustomed to seeing. Geothermal springs -- the result of volcanic activity -- provide hot, mineral-rich water for indoor and outdoor baths. Some are owned by communities, while others are attached to hotels or resorts. People visit onsen not only with their families, but also with friends and co-workers.
When it comes to onsen etiquette, Shinto, with its emphasis on purity and cleanliness, has an influence. Visiting an onsen begins with getting clean before you get into the water -- bathing is about relaxation and socializing only (and this pre-cleaning practice extends to bathing at home, too). Onsens always have showers, or at the very least faucets and buckets, and most provide soap and shampoo as well. It's unacceptable to get into the springs while dirty or even soapy.
Most onsens do not allow anything in the water but the bathers themselves, and this includes swimsuits. Everyone has a small white hand towel, which is usually placed on the head or by the side of the bath (although a few onsens do permit bathers to use their towel in the water). Many do not allow bathers with tattoos -- especially if they're large -- as these were traditionally associated with the yakuza, or mafia. There are usually different baths or specific bathing hours for each sex. Bathers are expected to slip gently into the water (no diving) and mill around quietly -- no swimming, splashing or rough play allowed. Many onsens have areas for napping, eating or just socializing after getting out of the bath.
In addition to onsens, there are also public bathhouses known as sentos, although they aren't as common today since many people have baths in their homes. Bathing at home doesn't provide the social aspect, but it's still all about the contemplative experience, not washing. After filling the tub with clean, hot water, it's used by multiple family members, who bath in order of age (more on Japanese bathrooms later).
This overarching sense of order -- even when socializing -- may make it sound like the Japanese don't have fun, but remember that these are the people who invented karaoke bars. If you're getting the idea that Japanese culture can be a bundle of contradictions, you'd be correct. Now, let's move on to the fresh, pure deliciousness of traditional Japanese food.