Kajiyama Motoyu at Kajiyama Yukawa Onsen is an extraordinarily rare cave carbonated hot spring located at approximately 670 meters elevation in the Nechi area of Itoigawa City, Niigata Prefecture. This hidden hot spring, positioned on the Fossa Magna (a major geological fault line), is truly the "secret hot spring among secret hot springs."

The history of this hot spring dates back to the Edo period, with records showing that in 1755 (Horeki 5), the people of Nechi applied to the Itoigawa domain for permission to use it as a therapeutic bathing place. The current cave is believed to have been dug after the Meiji era, with approximately 30 meters of horizontal tunnels excavated by hand. Once nearly forgotten, it was rediscovered and revived by the Nechi Future Conference around 2019, and now operates as the ultimate secret hot spring accessible only through guided experience tours.

The scenery features a mystical location surrounded by pristine forests along a mountain valley, where visitors can experience the echoing sounds of waterfalls within the cave. Near the cave entrance, hot spring water flows abundantly, and in some areas, reddish-brown mineral deposits (yu-no-hana) have accumulated up to 20 centimeters thick. As you venture deeper into the cave, the carbonation concentration increases, providing a fresher hot spring experience, though access to the deeper areas is restricted due to risks of oxygen deficiency and hydrogen sulfide poisoning.

In terms of popularity, it has gained legendary status among hot spring enthusiasts as "an incredible outdoor bath," attracting secret hot spring lovers from across the country. However, the restriction of access only through experience tours limits the annual number of bathers, further enhancing its rarity and value.