Beppu's one and only (as far as I'm aware) claim to fame is it's proliferation of celebrated onsen. The city and adjacent mountain villages overlie a rich volcanic spring and tourists and locals alike come to bathe in the springs and lap up the health giving benefits.
Onsen bathing is one of Japan's gems, put it on your must do list - you won't regret it....
Finding a mixed spa (if you prefer to take a bath together..) was tricky, most were separate. BUT you can hire private rotemburo (open air baths) for relatively little yen, if you're unsure 'bout exposing your bits to all and sundry of your own sex.
As with most things in Japan, there are a few golden rules.
1. You bathe in your birthday suit.
2. You wash before you bathe but more about this in a sec.
The first we tried was hidden up in the Myoban hills, the so-called 'hidden caves' (anyone who makes it up there without gagging surely has iron-clad nostrils. The springs radiate fumes of sulphur as a waste deposit, the aroma of rotten eggs was INTENSE)
Through the linen curtains and safely ensconced in your designated changing area, strip off and head for the washing area. Here a thorough scrub is called for. Any mingers diving straight for the spring unwashed will be swiftly reprimanded and sent back to the washing area. I tried my best to look like I was a pro and knew what the hell I was supposed to be doing but that's not all that easy when there's a coven of ancient Japanese ladies observing your every move as well as your milky bits.
Resisting the temptation to flee, I stuck it out, washed and then wandered casually into the murky green water, and then dived straight back out! I learnt a valuable lesson that day, that lesson being kids not to jump straight into lava heated water.
N.B once you are acclimatised, the hardest part is definitely prising yourself back out.
If you have triumphantly maintained a mask of casual nonchalence during your bathing experience then well done. You are on your way to a higher better place.
I know it'll come as a shock but there can be downsides to naked public bathing. Andrew had to sit through one guy's naked sauna exercises. Apparently the thigh squat was used to devastating effect. Retinal scarring aside, it's all good.