Dig your own hot spring in a riverbed
In a few places in Japan, hot spring water seeps up through the gravel of a riverbed. Dig a hole with a shovel, let the hot water fill it, and mix in cold river water until the temperature is right: an outdoor bath of your own making. Swimsuits are worn, and the baths close when the river is high.
In Akiyamago, a remote mountain valley in northern Nagano, you dig your bath in the gravel of the Nakatsu River, with shovels to borrow at the inn nearby. There is no direct bus; the last stretch is a shared taxi that must be booked, so it is easiest by car.
Hot water bubbles up through the gravel of the Oto River, 10 minutes by bus from Kumano Hongu Taisha on the Kumano Kodo pilgrimage trail. Bring your own shovel unless you are staying at an inn here. From December to February, the river is dammed into Sennin-buro, one giant open-air bath, and digging stops.