"Onsen" is my favourite word in the Japanese language. Translated into English, it comes out roughly as "hot-spring". Coming from the U.K., my previous experience of this heavenly pursuit was zero. Shortly after arriving here in 1991, though, I saw the light. A group of Japanese colleagues suggested we go to an 'onsen' one evening. As my Japanese was about as good as my Swahili at that point, I just agreed and tagged along at the back of the group. As far I knew, I could have been going to the pub, a movie, a brothel, a graveyard, a slaughterhouse, or anywhere. Within 5 minutes of arriving, though, I knew I'd found a new hobby. For those poor folks who haven't yet been for a dip, then get your kit off and plunge in. You won't regret it. Not unless you have a serious and very prominent defect on your body.
Onsens obviously vary enormously in size, shape and atmosphere. Some are little more than holes in the ground, out of which scaldingly hot water gushes, a few rocks lobbed in to resemble a makeshift bath. For many people, myself included, this is really the only way an onsen should be. Others (most), on the other hand, are (over-) developed resorts, catering to tourists by the train load. If you're thinking of visiting an onsen, then you may want to check to see if the phrase "Rotenburo" is displayed anywhere. This means open-air bath, and is generally a sublime experience, regardless of the season. I have sat in a rotenburo in Hokkaido in the middle of winter, air temperature around -20 degrees, and felt as if I were somewhere in the tropics. I would, in fact, recommend winter as the best time to go for a dip. You can appreciate the soothing warmth that much more deeply.
However, recreation is not the sole reason that onsens are as popular as they are. Depending on the chemical make-up of the spring water, a staggering array of ailments can be treated. Most onsens have a sign telling the fortunate visitor exactly what he can recover from. Everything from gout, high blood pressure, insomnia, heart disease and even alcohol poisoning can be eased by a trip to an onsen. I don't think one quick dip and a miraculous Christ-like cure is really on, though. Frequent immersion over many years (a pleasant thought) is really the only way.
Wherever you are in Japan, you're never far from salvation. There are several thousand spread liberally across the length and breadth of the country. Staying at an onsen hotel, though, can be a pricey affair. 30000 yen a night is not uncommon. However, most hotels will allow non-guests to use the bathing facilities for a nominal fee (generally 500-1500 yen). If you're really lucky, you may even stumble across a free one. They do exist, as do mixed onsens (i.e. men and women bathing together), but the number of these wonderful places is dropping year by year. Check the individual reviews for details as such.
Anyone with experience of Japanese onsens, please feel free to mail me at the address below with any comments/recommendations you would like included on this page.