Ipponmatsu Onsen (一本松温泉) is an onsen located in the mountains in Tazawako near Nyūto Onsen. It is a completely natural onsen, out in the open. It is a 30 minute hike from Magoroku Onsen (孫六). The water is very hot, perhaps more than 50°C (too hot to bath in!). There are however hoses running from the river to the bath, and if you can get the water to flow you can use that to cool the bath down to an appropriate temperature. It is completely free to use. As this area has a lot of snow in winter it is only accessible from late May to November.
This onsen is also called Tatsuko no Yu (たつこの湯), after Princess Tatsuko.
Access this onsen by first driving to Kuroyu Onsen (黒湯) car park. Drive through the car park and continue down the hairpin road down towards Magoroku Onsen (孫六). Drive over to the right and park in a safe area. There is a trail leading along the right side of the river, follow it until you meet a small bridge where you can cross the river. You will pass a beautiful dam along the way. Continue following the trail which will veer to the left to follow a tributary river. When the trail leads you down towards the river look carefully upstream and you will see arrows on the rocks, these will guide you to the onsen. Once you see those arrows you are within 100m of the onsen.
Access to the onsen during winter would be a difficult and very dangerous hike. Please visit this onsen between late May and November.