I went a few weekends ago to Kusatsu, a small hot spring town in Gunma Prefecture. Surrounded by impressive mountain scenery, it is located 1,200 meters above sea level and is constructed over a still-active volcano that releases sulfuric waters that never go below 50 degrees. Its main attraction is the Yubatake, at the very heart of the town, where hot spring water gushes down thick pinewood channels in order to cool it down to a tolerable temperature. Of course, there are many onsen close by. Sainokawara Rotemburo is perhaps one of the best I have ever been too (tatoo friendly, btw). Not widely known among tourists, I most definitely recommend giving Kusatsu a try if you come to Japan.