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Fuji-Hakone-Izu NP, Yamanashi
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Silkworm eggs were stored here in steady cold to delay hatching. Once warmed, the silkworms behaved as if spring had arrived and spun cocoons—letting growers raise silk beyond its usual season.
Before household refrigerators, summer ice kept in this cave had a market: the Japan Tourism Agency says it was sold to buyers from the capital or used to cool fevers.
The cave’s cold-storage career outlasted silk: after silkworm supplies, its chilled shelves also held tree seeds into the mid-20th century, helping preserve their ability to germinate.
At the far end, silver-blue patches shimmer on the walls. Nicknamed “glowing moss,” they are called keisanka by the cave operator; their shine comes from reflected light, not self-illumination.
A shout in Fugaku won’t bounce back like it might in a cavernous hall: its porous basalt walls absorb sound, so the cave famously produces no audible echo.
The cave preserves two very different lava signatures: partly molten wall material peeled away and curled into shelf-like rolls, while the deepest section holds rope-like ridges shaped by slow-moving lava.
The public route measures 201 m, but Yamanashi’s cultural-property inventory records 258.7 m for the cave overall—and notes a major side passage longer than 80 m.
Fugaku is a hollow legacy of Fuji’s Jōgan eruption, which began in 864: the cave is considered part of the long lava tunnels formed within the Aokigahara flow, beneath the forest.
Fugaku was designated a National Natural Monument on Dec. 17, 1929. Its official designation text notes that so many caves of this lava-tunnel type clustered around Mt. Fuji are globally rare.
