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Fuji-Hakone-Izu NP, Yamanashi
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Before Fuji’s 864 eruption, Senoumi stretched more than 8 km east–west. Lava filled its middle, leaving today’s Sai and Shoji lakes at opposite ends; Aokigahara now covers much of the vanished lakebed.
Aokigahara is called primeval, yet it grew only about 1,200 years ago on Fuji’s 864 lava flow. Soil is barely around 10 cm deep, so tree roots spread over the jagged rock instead of sinking far underground.
At Aokigahara’s eastern edge, Narusawa Ice Cave stored silkworm eggs from 1907. Its cold delayed hatching, helping local farms raise late-season silkworms—an underground refrigerator before electric ones.
At Aokigahara’s Ryugu Cave, a shrine honors the water deity Toyotama-hime. Local tradition says villagers prayed here for rain in drought; Fuji pilgrims later made it a sacred stop on their Eight Lakes circuit.
In Aokigahara’s Saiko Bat Cave, temperatures stay 8–10°C year-round. Five bat species have been recorded there; one December count found 444 lesser horseshoe bats hibernating in this protected lava-tube refuge.
Some lava tubes beneath Aokigahara run as far as 4 km. The well-known Fugaku Wind Cave and Narusawa Ice Cave are thought to be exposed sections of longer tunnels, left behind as molten lava drained away.
Airborne laser mapping revealed a previously unrecognized vent, 下り山火口, beneath Aokigahara’s canopy. It is the northernmost Fuji crater known to have erupted within the past 3,200 years.
When the 864 lava spread across the plateau, it flowed around small hills, leaving unburied patches called kipuka. Many remain near Lake Motosu’s eastern shore—little islands of terrain amid the lava.
A curved, mossy wall built from Fuji lava runs through Aokigahara, reaching about 1.8 m high. Its date and purpose remain a puzzle even to local cultural-property specialists.
Compass needles work normally at hand height in Aokigahara; the lava can deflect one only when it is held directly against the rock. The real challenge is the uneven terrain and lack of obvious landmarks.
In one research plot, the oldest hemlocks were about 330 years old, compared with about 220 years for hinoki cypress. Researchers concluded that hemlock colonized the lava first—the forest’s tree mix is still evolving.
