AT A GLANCE
Why go?
Tucked off a prefectural road in the deep mountain village of Oshika (大鹿村) in southern Nagano, Okeya-no-izumi Spring is a small, roadside natural spring where locals and passing travelers stop to fill bottles and tanks with cold, clear water straight from the ground. There is no ticket gate and no visitor center here — just a stand at the mouth of a seep, the sound of running water, and a quiet gorge setting that makes the pause feel like a genuine slice of rural Ina life.↗↗
What gives the spring its character is the unusual geology of the whole valley. Oshika sits along the Median Tectonic Line (中央構造線), one of Japan's great fault zones, and the rock changes abruptly at the fault. On this side, around the white-granite country of the Kosubu River gorge known as Kosubu-kyō (小渋峡), the spring water runs as a soft, near-pure mineral water — a pointed contrast to the calcium-rich hard water that seeps from the greenstone terranes on the other side of the line.↗
A spring defined by the fault line beneath it
The people who interpret this spot are not a tourism office but the Oshika Central Structural Line Museum (大鹿村中央構造線博物館), a striking geology museum set in the same village. Its materials frame Okeya-no-izumi as one of the valley's two showcase springs — the other being the nearby Firefly Spring (蛍の泉) — chosen precisely because they illustrate how water tastes so different on either side of the fault. Where Firefly Spring rises from limestone-and-greenstone terrain and comes out hard and mineral-rich, Okeya's water issues from pale granite, so it is soft, clean, and close to pure.
