The finest open-air onsen I have ever visited is not inside any luxury ryokan. It sits beside a mountain hut at 2,011 meters, partway up Mt. Hakuba Yari in Japan’s Northern Alps, and the only way to reach it is on foot. Hakuba Yari Onsen is a free-flowing natural hot spring — the water rises straight out of a fissure in the rock and pours into the bath — and you soak in it while looking out across the peaks of the Northern Alps. This is not a spring you drive up to; it is a hidden onsen you have to hike in to reach. But if you love hot springs and you can walk this far, the feeling stays with you for a very long time.
The highest natural spring source in Japan
Yari Onsen sits at 2,011 meters, partway up Mt. Hakuba Yari. The water is a bicarbonate spring at 43.1°C, rich in sulfur and minerals, carrying a strong sulfur smell. It rises from a fissure in the rock beside the hut and flows straight into the bath, with no water added and nothing blended in. By elevation it ranks as the third-highest alpine hot spring in Japan, after Mikurigaike Onsen at Murodo on the Tateyama Kurobe route (2,410 m) and Honzawa Onsen (2,150 m). But as a spring source it stands alone: its source is the highest naturally gushing hot spring in Japan, it is the hardest of the three to reach, and for the sheer scale of the view, it has no rival.
The hike: doable, but you have to cross snow gullies
This is a hiking route, not a spot for casual sightseers to wander up to alone. From the Sarukura trailhead (1,245 m) to the spring (2,011 m) is 5.6 km with about 800 m of ascent; a regular hiker needs 4 to 5 hours up and 3 to 4 hours down. As mountain routes go it is moderate: there is no dangerous terrain on the climb from Sarukura, and even someone who does not hike often can usually manage it with a bit of fitness work and an experienced companion along.
One thing to flag for foreign hikers: the trail crosses three snow gullies (steep gullies where snow lingers into summer), and it is easy to lose sight of the route on the snow, so watch it carefully — we lost our way on one of them. If you continue up toward the summit of Mt. Hakuba Yari, there is also a short exposed section equipped with chains not far above the onsen. By Taiwanese standards it is not especially hard, but it still calls for care.
How to get to Yari Onsen
First get to Hakuba village. From Hakuba Station or the Hakuba Happo bus terminal, take a bus or taxi to the Sarukura trailhead, and start walking from there.
Staying at the hut
The hut is put up only for the summer climbing season and taken down entirely once the snow comes; it belongs to the Hakubakan group. Inside there is a communal, dormitory-style sleeping area with space for 100, plus 30 pitches at a campsite. A half-board stay costs ¥17,000 per person, and you get a mattress, futon and pillow, a small shop and a simple dining room. Apart from the hot spring, which comes straight out of the mountain, everything here — food, fuel, supplies — is flown in by helicopter once every two weeks.
By Taiwanese mountain-hut standards this place is well equipped, even luxurious: charging and meals are no problem. By Japanese standards, though, it is a “simple hut” — there is no drying room, a facility found in most Japanese mountain huts, so wet clothes have to be hung along the outdoor corridor. In damp weather they barely dry, and you may well set off the next morning still in wet gear.
The bath and mixed bathing
There are two baths. The main one — and the famous one — is the open-air bath with its sweeping mountain views, and it is mixed-gender bathing (konyoku); for one to two hours at night it becomes women-only. The second bath is an indoor bath that is women-only the rest of the time. In the open-air bath you can wear a swimsuit or go bare. I had read other visitors’ accounts beforehand and was a little unsure how it would feel, but the weather was poor when we arrived and only five or six of us had come up, so my son and I had the place almost entirely to ourselves for the whole stay. The staff only reminded us of the evening women-only window and gave us no other special instructions. In practice, by day the open-air mixed bath is open to everyone in principle but tends to be a men’s affair.
The bath happens to face east, so with luck you can soak here and watch the sun come up over the ranges. Our weather was bad and we never got to see the sunrise, but the experience was still magical — sitting in warm water at an air temperature of 5 or 6°C is a pleasure in itself.
Is it worth it?
We reached the spring on a three-day, two-night traverse of the Hakuba Sanzan — the three peaks of Hakuba — dropping down from the summit ridge to Yari Onsen on the second day to stay the night. We chose the longer route largely because I had wanted to visit this onsen for years. After two days of rain — cold, soaked to the skin, and briefly lost on a snow gully — sinking into a pool of free-flowing spring water with the whole of the Northern Alps spread out below: against that, even a top-end ryokan at tens of thousands of yen a night could hardly compete.
