Yes, you read the subtitle right. It’s a bold claim, but until I find an onsen that better meets my criteria for what makes an onsen great (remote outdoor location surrounded by nature, real hot spring water from the ground not a manmade bath, construction that works to blend into the landscape [or better yet, no manmade incursion of any kind], and thin to non-existent crowds), Kappa no Yu is my best.
Located in the Yagen Valley in the mountains near the top of Aomori Prefecture’s Shimokita Peninsula, Kappa no Yu’s location is as remote as it is possible to be without leaving Honshu. It’s tucked away in the side canyon of a tributary of the Ohatagawa River, the pool itself appears to have been - for the most part - hewn out of the rocky stream bank just a few feet above the clear waters of the tributary river below, and the wood and bamboo roof that shelters the pool from the worst of the elements all but disappears into the surrounding forest of deciduous trees. I can only imagine the glory of the place in the fall. The water in Kappa no Yu comes straight out of the ground and is the clearest blue of any hot spring I’ve yet seen in Japan, and as if this cake didn’t have enough icing and cherries on it already, there’s no entry fee, apart from what it costs to get you there, and the Yagen Valley is functionally uninhabited. If you go to Kappa no Yu, you’ll probably have it all to yourself. I did.
There is no train station in the Yagen Valley and only one bus per day (at 8:40 in the morning) from Mutsu City via Ohata-Machi. A car is is highly recommended and, while there are noodle/souvenir stores in the valley, you’ll need to bring your own supplies. Camp at the Yagen Campground (Yagen Yaeisho) for ¥500 or check into the Yagenso Minshuku (¥4500/night no meals, ¥8400/night with breakfast and dinner, open April-November) or the New Yagen Hotel (from ¥14,700/night meals included).The best hot spring I’ve ever been to is Mt. Princeton Hot Springs in Colorado. It is just a few pools of varying temperatures in the sand and rocks on the bank of Soda Creek at the foot of Mt. Princeton. Kappa no Yu isn’t quite that but it's my solid second place. The countryside surrounding Kappa no Yu is wilder even than Colorado. If you really want to get away from it all, you can’t do much better than this, and if you know of a better onsen than Kappa no Yu, I want to know where it is. Happy Travels!