In the mountains between Nagano and Gunma this little onsen town keeps warm with numerous hot springs and many ryokan and hotels. Most hotel/inns will have their own but there are several very nice public onsen around the town.
A short walk through the town to the West is the nishi no kawahara outdoor bath. It has a huge outdoor bath area. It's beautiful and steamy in midwinter.
To the East, downhill is Ootaki no yu which is a mostly indoor bath. This onsen has two interesting bath types which alternate between male and female access. The general idea for both is that there are many wooden baths with different temperatures which you move between in order. One is stepped with baths running into the one below it.
They're both really interesting and worth checking out if you have time for both.
In the centre of town is a large onsen pool where the boiling hot water bubbles to the surface leaving sulphurous deposits and filling the area with steam.
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