Just 30 minutes by direct train from Yufuin and 90 minutes from Hakata Station (in Fukuoka) via the Yufuin-no-Mori train lies Amagase Onsen near Hita, the closest major city. Amagase Onsen is one the "Bungo Major Three Hot Springs (Bungo Province is now what is known as Oita Prefecture), along with Beppu and Yufuin, and is best known for its mixed bathing open-air baths along the river (called Kawayu.) There are several stores in he area that sell "Yuami" clothing, which are water wear that's commonly used in mixed gender baths that allow coverups such as the ones in Amagase Onsen. While not well known among Westerners, Hita and Amagase Onsen do attract a fair number of Asian travelers due to its accessibility. Hita and Amagase Station are along the Yufuin-no-Mori train route that runs from Hakata Station in Fukuoka to Yufuin, so it's many visitors spend a few days in Fukuoka, then traverse through Hita, Yufuin and Beppu, and fly out of Oita Airport or return to Fukuoka.
Aside from enjoying the Kawayu onsen baths, there are some other things to do here. Mamedamachi is a very nicely preserved old town center in Hita, with several old merchant homes now operating as museums, cafes, restaurants and stores. It's not as large as Takayama, Kanazawa or Matsumoto, but it's not overrun with tourists, either. We noticed some Asian tour groups roaming the streets, but it's relatively undiscovered by independent travelers. For those of you interested in Hina dolls, the museum in Mamedamachi is the most impressive one we've ever seen, with awesome displays of the dolls from different eras. Also in the town center is Tezukuri Ohagi Inaka-an, a cafe that serves wonderful Japanese desserts, including Ohagi - sweet rice cakes that are similar to Mochi but quite different in texture. From mid-February to mid-March, Hita really comes alive, as plum blossom viewing reaches its peak during this period. We checked out the Hita Oyama Plum Festival a few years ago (on our way to Beppu), and it's definitely worth experiencing if you happen to be in the area at that time.
Sanso Tensui is the top property in the greater Hita region. If our experience there is any indication, I'd argue that it's easily among the 20 best ryokans in all of Japan, and priced at a fraction of the rates that top properties command in nearby Yufuin. Established in 1998, this 19-room ryokan is surrounded by a moss forest, is located next to a gushing river and impressive waterfall, and the smell of the onsen is just as strong (if not stronger) than the vast majority of hot springs we've experienced. The architecture is traditional rural Japan (it's very charming), but the interior is Taisho-era (similar to Iwanoyu), which isn't to our tastes but it fits really well with the atmosphere here. It's also far enough away from the main part of town that you'll really feel secluded when staying here. The area has a large number of massive boulders, and Sanso Tensui's architects did a masterful job of building the structures around the boulders in an effort to preserve the natural environment. The owner and his wife also own Kaya Usagi (also in Hita) as well as Kurokawa-so in Kurokawa Onsen, a ryokan that we wouldn't mind visiting one day.
We stayed in "Special Suite Kaori", which is an 85m² accommodation and one of the three "special suites" that were created in 2017. All of the special suites feature two (or more) rooms, a spacious terrace, an open-air Onsen as well as an indoor Onsen. Although it's not to our personal tastes, here, the Taisho-era furnishings and decorations gave it a soothing sense of place, and we could tell they put a lot of (unique) thought into the rooms. The bathroom sink uses the remains of an old Singer sewing machine stand, which is a cute touch, and the overall eclectic mix of early 20th Century Japanese and Western aesthetics were interesting without getting in the way of comfort. On the deck of our room was a large, open-air onsen rock bath with views of the forest and the sound of the gushing waterfall (which is only viewable from one of the public onsen.) The only drawback was the bathroom only having one sink. Note that even with the doors closed, the onsen smell (caused by hydrogen sulfide in the spring water) is acute, and may prove to be overpowering to some people. Furthermore, the sound of the river flow and waterfall is extremely audible - if you are sensitive to strong odors or sounds, this might hinder your ability to sleep. For people with such concerns, I suggest you book one of the 5 rooms that do not have onsen.
There are some wonderful bathing options elsewhere on site, beginning with a whopping three different Daiyokujo (gender-segregated public baths.) Takimian is accessed by climbing a long wooden staircase up the side of a hill, and the open-air onsen offers scenic views of the river and Sakura Falls. Hibiki-no-yu (for women) and Take Sansui (for men) are massive rock baths that are set along a picturesque river. Finally, there is an indoor public bath for those that don't want to brave the Winter cold before getting into the onsen.
Then there are five private rental baths that are clustered in the far end of the grounds, all of them face the Goraku River. The most interesting ones are Kamayu, which is a small bath (only big enough for 2) shaped like an Udon noodle bowl and is an onsen jacuzzi, Kawayu and Kiriyu, both of which are big enough for 4 bathers (Kiriyu is slightly larger and might be able to fit more.) These are all free to paid guests and is available for use on a first-come, first-served basis.
For dinner, all three Tokubetsu Shitsu (special rooms) have meals served in their own room, and the same goes for the 5 rooms that do not have in-room onsen - all others dine in the ryokan's main restaurant, which isn't a major negative since all guests at the restaurant get their own koshitsu (private room.) Breakfast for all guests are served at Sanso Tensui's café lounge Hakurakuten. Both meals were incredible; we loved the Yamame Zenryufun-age (Japanese Cherry Trout from Oku Hita fried with whole grains), which had a nice, crispy (but light) texture and juicy inside, the Kumoko Chawanmushi (savory egg custard with Cod Milt) was dreamy creamy, and the Kasu-jiru (Soup dish made by simmering Sake lees) mixed with seafood and vegetables was an awesome substitute for the typical miso soup at the end (together with rice and pickled condiments.) And the Kusu Hitomebore rice, served at both dinner and breakfast, was delectable. Dinner was elegantly served by Yuko Baba, who patiently explained all servings in painstaking detail.
Sanso Tensui really punches above its weight - it's a real bargain, and the onsen experience is second to none. Despite being off-the-beaten-path, it's also very accessible - Unless you're dying to check out the "Hells of Beppu" (for mesmerizing experiences go to Iceland or Yellowstone instead!), I'd definitely suggest a combination of Yufuin and Hita (or Kurokawa Onsen) versus heading over to Beppu.
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