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Fuji-Hakone-Izu NP, Yamanashi
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From 1907, the cave kept silkworm eggs cold until farmers needed them. Narusawa’s history records 19,850 special-grade egg sheets stored here in 1914; this cooling system helped make late-autumn silkworm raising possible.
Unlike the familiar horizontal lava tunnels nearby, Narusawa drops vertically through 2 ring-like levels. Its 153 m route reaches 21 m underground, and one passage has a ceiling just 91 cm high.
The cave’s ice pillars grow as water seeps from the ceiling and freezes. Around April, they can reach 3 m tall and 50 cm across in some years—a towering result of tiny drips.
This cave formed when lava flowed from Nagao-yama, a volcano on Mount Fuji’s flank, in 864—not from an eruption at Fuji’s summit. It was designated a Natural Monument on December 17, 1929.
A local legend says the cave’s deepest “Hell Pit” runs underground all the way to Enoshima Island. It’s a story, not a proven passage: Yamanashi’s tourism guide explicitly calls the claim unsubstantiated.
Among the lava and ice are the remains of a real well, used before water service reached the area. It’s an unexpectedly everyday trace of village life inside this dramatic underground landscape.
