Visited Futamata Radium Onsen – One of Only Two Siliceous Sinter Domes in the World!
Travel Date: November 2023
I finally made it to Futamata Radium Onsen, one of those hidden hot springs in Hokkaido I’d been dying to visit! It’s famous across Japan for being one of only two places in the world where you can see a natural limestone sinter dome—the other one being Yellowstone in the U.S.
Futamata Radium Onsen is located deep in the mountains, about 10km from the town of Oshamambe, then another 10km along a mountain road. The star attraction? A 25-meter-high dome formed by natural deposits from the spring water—it's absolutely wild to see in person!
This rare hot spring is rich in natural radium and calcium, and has long been known for its healing properties. It's also a popular spot for long-term therapeutic stays (known as toji), and the hallways are lined with testimonials from guests who claim to have improved their health.
Oh—and when I parked my car, I was greeted by three wild foxes! At first I thought they were dogs. Classic Hokkaido moment.
The front desk was unmanned. Day-use guests are expected to buy a ticket from the vending machine and drop it in the box—a very DIY style. Since I was staying overnight, I shouted “Hello?” into the hallway, and the hostess came out to greet me.
She seemed to be running the entire place herself, but was full of energy and kindness. She even checked in with me during meal times. As a solo traveler unfamiliar with the flow of things, her presence was very reassuring.
My room was basic but cozy—exactly what you'd expect at a traditional toji inn. No bath towels are provided, but you get a small towel and toothbrush set. You can also buy beer and soft drinks from the vending machine, which was a nice touch.
From my room, I could see the edge of the sinter dome through the window. The colors were amazing—got me all excited for the baths!
There’s a small indoor bath and a small women’s open-air bath. The main large outdoor bath is mixed-gender, which made me a bit hesitant—especially during the day when there were a lot of male guests using it. Honestly, I couldn’t bring myself to enter...
From the women’s changing area, the mixed bath is completely visible. But I waited until late at night when there was only one older man left and made my move! You have to pass through the men’s changing room to get to the bath—major hurdle—but it’s dark at night, and fewer people are around, so I gave it a go.
The older gentleman said, “Don’t worry, regulars don’t mind at all.” LOL
I had hoped for some stargazing from the bath, given the remote location, but sadly the sky was overcast. Still, I’d love to come back someday and try again.
They also have a small hot spring pool, though no one was using it. Maybe kids swim there in summer? The mineral quality is the same across baths, so honestly, the women’s bath is totally fine too.
The outdoor water was warm and relaxing—not too hot—so you could soak for a long time. I noticed tiny bubbles clinging to my skin, and the bath itself was crusted in mineral deposits. It’s not a big bath though, so I was careful not to linger too long during busy hours. Toji guests tend to take loooong soaks.
The indoor bath is hotter and great for warming up. It had one deep tub and a regular one. The deep one reminded me of the baths at Fujiya Ryokan—totally different vibe and water, but still nostalgic.
Amenities and showers are nonexistent here. I tried to wash my hair with just the hot spring water and… yeah, that was a bad idea 😅
There’s a little tub where heated water is collected, and plastic bottles float in it. Apparently, you can use that water for rinsing or washing your hair.
The water is a sodium-calcium chloride spring that contains natural radium. It tastes salty and slightly metallic.
Depending on the lighting, the bathwater changes color—from brownish to greenish to bluish hues. Early in the morning, I noticed a thin calcium film forming on the surface. Wild.
The website says:
“If you stay for more than two weeks and feel no improvement, we will refund your stay.”
That’s how confident they are in the healing powers here. Honestly, the water was so strong that if you soaked for two weeks straight, something in your body would definitely change—for better or worse.
Meals were classic toji fare—simple and healthy. I felt like if I stayed here, ate this food, soaked in the baths, and took walks every day, I’d lose weight and feel amazing.
Personally, I really enjoy the rustic charm of toji inns, so the slightly outdated facilities or relaxed service didn’t bother me at all. But if you’re expecting full-on ryokan luxury, this probably isn’t the place for you.
Still, the baths were incredible. If it were closer, I’d definitely be a repeat guest. And I’d love to see those foxes again.
Just my honest thoughts 💭
Some hot spring photos were taken from the web.