Hirauchi Kaichu Onsen (Underwater Hot Spring)

Amazing experience walking down to this onsen at 8pm (low tide was at 10) on a Sunday night, and sharing the place with a few locals. There was an electrical storm on the horizon and the pool was not too hot. Incredible being in warm water while waves rushed in towards us (but never hit us) and watching the lightning on one horizon… and looking up at the stars on the other side of the horizon….unforgettable…

Fascinating unusual onsen. Mixed nude bathing. But you can also have a simple foot bath in one of the the small shallow pools at the end of the walkway. Although the signs refer to lockers, bring a bag to put your clothes in when you go for your onsen and store your gear on the nearby rocks. Take a basin or ladle to wash yourself in sea water near the onsen. With high levels of calcium in the water it is very slippery, so take your time and be careful. As the tide slowly comes in it is fun to move to the small pools closer to the sea and get washed back in.

Very cool place. Highly recommend. The low tide that day was around 2pm; we went around 4:30pm and it was still very good. Almost everyone was local. No one was wearing swim suits, but all the men and women were using towels to cover their bodies. I unfortunately didn’t bring a towel, so I went in fully naked… The people were respectful, but I was worried that I offended them… oh well. Before you go in, grab a little basin and wash yourself with the water from the two smaller pools.

Would have been nice, however despite there being signs that say 'no clothes' and 'no cameras' (it is a mixed onsen with only onsen towels allowed), there was a lady using the onsen like a foot bath, another couple in the onsen fully clothed who took photos on her way down the ramp (towards the baths, past the 'no photos sign') which understandably would have made any naked bathers feel uncomfortable (hence the rule and the sign!) My girlfriend had already stripped down as per the instructions and she wanted to go in to experience the mixture of warm onsen water and the chill of the sea water, but given that multiple people were breaking the rules, the whole thing felt 'off' so we left without getting to have the experience.

Beautiful rotenburo (outdoor onsen)! Views are great, the temperature of the different pools vary, so you should find one that's just right. Make sure you check the times for low tide and remember this one's a mixed bath and no swimsuits allowed. Women can wear a bathing towel (like a cloth dress), see photo. Honestly, it's a little intimidating being so exposed as the path to the onsen is elevated so people can see you from 50m away and there are a few occupied houses in very close proximity, but if you can get past that, it's great!

I didn’t end up going in as it was (a) too hot a day and (b) only men there when I went. But it looked like it would have been nice

Very nice natural hot spring. Easily accessible by car. It says everywhere that swimwear is prohibited, but almost all women where wearing it together with a towel. All men bathed naked, but to bring a small towel seems to be the thing to do, to slightly cover yourself when sitting on the side to cool down, or walking around.

excellent experience - Soak in warm water while looking at the crashing waves. Can’t wear swimming costume but women can wear towel dress or even just a light cotton regular dress. Men take a towel to go round your waist. Notice says best experienced 3hrs before low tide to 3hrs after low tide. Google to find low tide. Actually liked it 4 hrs after low tide - waves start coming into the hot pools - you get the mix if hot volcanic water with warm seawater.