There are two types of visitor to Japan: those who’ve never visited an onsen, and those who, having visited one, end up slightly obsessed with visiting more and more of them. Having stumbled into the latter camp on my very first trip, I’d say an onsen (or two, or ten) is an unmissable part of any visit – here’s why.

Bath to Basics

A quick explainer is probably in order. In the UK – and, as I understand it, the US and most of Europe – a bath is a place to get clean. You run some water, get in, wash yourself, and get out, normally discarding the water immediately. In Japan, things are a little different. You get into a bath only after getting clean, soak for a while, then get out – usually keeping the water, which should have remained clean, for others to use – it’s disposed of at the end of the evening. Or, sometimes, pumped into a washing machine for further use.
If traveling on the cheap you’ll probably encounter this kind of bath once in a while – in older ryokan or minshuku, or youth hostels. Just remember to wash before you get in – never use soaps etc in the bath – and cover it when you leave. Easy.
Today, most homes have a bath. In the past, though, this wasn’t the case – so residents would use a sento, or public bath. The principle of this is exactly the same: go to the sento, wash first, then bathe, then leave. You can still find sento all over Japan.
An onsen is a cross between a sento and what we might call a hot spring – a public bath where the hot water is drawn straight up from the ground. Some will tell you they have medicinal benefits; I say that’s probably nonsense, but they’re certainly one of the most enjoyable ways to pass a few hours after a long day’s travel.

Finding Onsen

Because they require a hot spring, onsen are often found clustered together in volcanically active areas where such water is plentiful. And because going to an onsen is a leisure pastime, many such areas have developed into resorts where people might go for a weekend to stay in a nice ryokan, bathe in onsen, and generally chill out.
Onsen resorts I’ve visited and can recommend include Hakone, Nyuto Onsen in Tohoku, Kinosaki in Kansai and Beppu on Kyushu (which is pretty much onsen, and nothing else) and Toyako on Hokkaido. But you’ll find others scattered everywhere, and some of my favourite onsen have been in less resort-y places: on Sakurajima volcano and near Aso-San, for example. Chances are that wherever you go, there’ll be an onsen town somewhere nearby.
In fact, even if you don’t leave Tokyo it’s possible to check out an, albeit rather strange, onsen experience: head to the O-Edo Onsen Monagatari (pictured above) on Odaiba. This slightly mad place is half onsen, half theme park – but ignore all that and you can still chill out in an outdoor bath, watching the planes zip over the Tokyo bay to Haneda. It’s become my jet-lag busting habit whenever I pass through the capital (go late to beat the crowds and get in cheap).

Onsen By Numbers

So you’ve found an onsen – what next? Here’s how it works.
1) Don’t forget your Towel. If you’re visiting an onsen just for a bath, rather than staying in a ryokan with onsen attached, it’s best to go prepared: take a towel to dry off with, a small towel-slash-cloth-thing (if you have one), and some soap. You’ll only need the soap in the most back-to-basics places – most midrange, and all upmarket onsen provide soap, shampoo and the like – but you’ll need the towels in most places, with the exception of posh upmarket ones. Nonetheless, you’ll never regret having packed an unnecessary towel, so go prepared.
2) Pay at the front. The cost of entry to an onsen will range form 100 yen (rare) to well over 1000yen for a posh location, and you pay in the way in. 300-500 would be an average price. The exceptions to this rule are the really strange onsen (like O-Edo Onsen Monagatari) in big cities, where sometimes you pay on your way out, having racked up a bill – in this case you’ll still need to check in at a front desk. If you don’t speak any Japanese, I’d imagine just holding up fingers for the number of people who want to enter would do the trick.
3) Look for Lockers. Having paid, are there coin lockers in the front desk area? If so, lock any bags, wallets or other valuable stuff away in here.
4) Split Up, and Get Changed. Having paid, look for the curtained doors that lead to the changing rooms. These will be segregated, with one for men and one for women, as you’d expect.  Often you can tell which by colour coding: blue for men, red for women – or look for the kanji: 男 for men, 女 for women. In many cases the curtains will have writing on them, but it will read only ゆ or 湯, which means “bath”. If in doubt, do ask. For men: “o-toko wa dochira des ka” means, roughly, “which one is for men?”. For women, use “on-na wa dochira des ka”.
If you didn’t spot any lockers at the front, the chances are there will be some here – but not always. In many cases you’ll find a load of big baskets instead. Leave your big towel, and all your clothes, in one of those. If you can see things like free razors, shaving cream, soap etc lying around near a basin, you can almost certainly abandon your own soap, too, as there will be stuff provided in the baths.
And here you reach the sticking point for some people: you need to get naked. No swimming costumes, or any clothing, should be worn into the baths. Take it all off – the only things you should carry into the bath are a small towel/cloth thing, maybe some soap if it’s a really cheap place, a locker key if you have one and your glasses if you need them to see (if you can do without, leave them behind too).
Almost all baths are segregated by gender, like the changing rooms – but you will, very occasionally, come across mixed ones. On the single occasion I visited a mixed bath, I didn’t actually realise until afterwards – it was full, as usual, of old guys talking about the news.
6) Wash Up. Head into the bath area, but not straight into the bath. Instead, look for the big row of sinks, normally with plastic buckets (and often with provided soaps, etc). Head over there, and wash yourself conspicuously from head to toe. Usually there are showers, but if not use the plastic bucket to dump water over yourself. You might occasionally spot old men skipping this step, and just dashing a single bucket of water over themselves – I’m not sure that’d go down well at all if you tried it as a foreign visitor.
7) Finally, Get In! At last, you can get in the bath. This probably doesn’t need to be said, but just in case: don’t jump in, or anything like that. Get in, sit down, and wallow in the hot water. A word of warning: most onsen I’ve met are hot, but not unpleasantly so. Once in a while, though, you’ll meet one that seems to be boiling you alive, so take care. Oh, and remember your little towel? Don’t dunk this in the bath, but instead fold it up on your head (you’ll see how it’s done).
8) And Relax. That’s it. Chill out. There might, if you’re lucky, be a few baths, so check them out – sometimes you’ll find baths with seasonal fruit in them, or freezing cold baths, or murky baths filled with mysterious “black” spring water – but in any good onsen I’d look for the “rotenburo” – an outside bath, and in my opinion the best thing ever. How long you find it pleasant to stay in an onsen will vary. You’ll often see people getting out, sitting around, then going back in – there’s no obligation to cook yourself for longer than is comfortable.
Onsen are sociable places. Sometimes you’ll find people soaking in silence, but more often you’ll see people chatting away. If you speak a bit of Japanese, or if anyone else speaks a bit of English, there’s a good chance you’ll end up involved in one of the conversations, which can be nice – I’ve ended up in some quite splendidly random discussions, many of which revolved around the Beatles (I know nothing about the Beatles, and occasionally think I should remedy this purely to help out when chatting in Japan.)
9) All Done? Once you’re done bathing, have another quick wash before you head out – I don’t think there’s no risk of offence if you skip this, but you might stink of sulphury water, yuck. Then head out and get changed.
10) Have a Beer. Optional but recommended. You’ll sometimes see vending machines selling cold milk in the onsen changing rooms – this is nice, but what’s even nicer is to sit outside the onsen, out in the countryside somewhere, and drink a beer. Just be careful not to pass out; the hot water can make your brain a little wonky.
Honestly, this might all sound like a huge amount of faff just to get into a big pool of hot water, but it’s the best thing.