With each of its eruptions over the centuries, Mount Fuji has changed the surrounding topography. After the Jogan eruption in the year 864, lava hardened and formed a base where a “Sea of Trees” called Aokigahara Jukai would eventually develop on the northwestern lower slope. The forest now extends over a huge area—about 3,000 hectares—between 900 and 1,300 meters above sea level.

Trees grow in abundance and the forest seems practically impenetrable. It is so thick that, in the old days, people found something fearsome about the place, with rumors that people who found their way in would never come out again. This may explain why very few people—even the local Japanese—set foot in it. But that is changing with a recent appreciation of the forest’s remarkable natural environment, and eco-tours there are now on lists of easily accessible destinations. Kuribayashi Shuki is a nature guide registered in the town of Fujikawaguchiko, and he will now take us into the forest.

After a lava field forms, the first things to grow are lichens (organisms made up of fungi and algae). Later, moss appears, then grass, and these plants build up the soil little by little. What sets this forest apart from others in the world is that, even though the soil is only about 10 cm thick, it supports the growth of trees. This is possible because of the Pacific Ocean, lying not far off to the south. The air blowing in from the ocean is humid, creating an ideal environment for moss, and that moss holds plenty of moisture for the trees to grow.

About 80% of the trees here are hinoki cypress and southern Japanese hemlock, both evergreens. In the shallow soil over the hard lava, their roots must spread sideways, sometimes rising above the ground to make humps and hollows. After the trees reach a certain height their roots can no longer support the trunks, and they fall over. So they are mostly around the same height.

Left: Water from the cave ceiling freezes as ice “stalagmites.” These can be seen throughout the year.
Right: The transparency of the ice indicates that the water seeping into the cave from the mountain is very clean.

The Fuji Fuketsu wind cave, formed when the lava hardened. Deep inside the cave there is no light, making it too dangerous to enter without a permit and a guide to lead the way.

Under the tree canopy, the air is oppressively humid and the trees give off a noticeable fragrance. They grow thick overhead, making an umbrella unnecessary in a light rain. This is a great place to enjoy a “forest sauna.”

We walk about 30 minutes from the trailhead and come to the Fuji Fuketsu wind cave. It is more than 230 meters long and, in places, runs as deep as almost 20 meters below ground level. Like other wind caves, it was formed during a volcanic eruption, when gas in the lava burst through, leaving a cave or tunnel as the lava cooled.

There is some ice inside, even in summer. The locals say that in the early Edo period (17th century) the Shogun, Tokugawa Ieyasu, had ice brought from wind caves like this one to the metropolis of Edo (present-day Tokyo). We grope our way down into a pitch-black hole and, about 30 meters inside, see ice on the walls and other surfaces. The ice crystals developed from underground water that continuously seeped in from Mount Fuji. The freezing temperature is due to the fact that evaporation is a cooling process—when water in the cave evaporates, the ambient temperature drops. The thermometer registers -2 °C in summer, -15 °C in winter.

We find our way out of the cave and then climb nearby Mount Omuro. Right away the ground feels different under our feet. In Aokigahara Jukai, the lava bed was readily visible and the footpaths were hard, but here on Omuro, broad-leaf trees like Japanese beeches and Mongolian oaks rule—their dead leaves and soil have accumulated over time, making a soft trail bed. And the sun shines bright through the trees.

“I really like Mount Fuji. I wanted to learn more about it, so I began working as a guide,” says Kuribayashi. “The Aokigahara Jukai is part of Mount Fuji. The forest’s wonders and its ability to thrive and mystify can be experienced during an eco-tour, and I hope many more people will come and see for themselves.”

Aokigahara Jukai evolved under the influence of Mount Fuji. Our tour has shown us the beauty of an old growth forest, where nature offers plenty of wonder.

Trees in Aokigahara Jukai must extend their roots along the surface. The most common trees here are hinoki cypress and hemlock.

Photos taken very close to Aokigahara Jukai on Mount Omuro (elevation: 1,468 m), a volcano on the side of Mount Fuji. Fuji has many such flank volcanoes.
Upper left: Immense trees on Mount Omuro have been growing for three or even four centuries.
Upper right: Kuribayashi Shuki is a nature guide registered by the town of Fujikawaguchiko. When he talks about Mount Fuji’s history and the unique characteristics of the trees, his eco-tourists find him easy to understand. “A world without trees would be a world without humans. We need to take better care of forests—that’s the theme behind my explanations.”
Two photos below: Mount Omuro is clothed with many types of trees, such as the deciduous Japanese beech and Mongolian oak, and a conifer originating in Japan called tiger tail spruce. A thick crimson glory vine used as a swing—a gift of nature.

Map of Aokigahara Jukai “Sea of Trees” and environs

The Fuji Fuketsu wind cave is prized phenomena in an untouched natural world, and you need a permit to enter it. (Join an eco-tour, and the tour organizer will apply for a permit for you.) To facilitate tours, infrastructure for tourists has been added at the Fugaku Fuketsu wind cave and Narusawa Hyoketsu ice cave.

For more info
Fuji Eco-tour Service (Japanese language website) introduces tours to Fuji Fuketsu wind cave and Mount Omuro, and ecotours up Mount Fuji:
http://www.fuji-eco.com/

Fujikawaguchiko Sightseeing Information Site (in Chinese, English, Japanese and Korean):
http://www.fujisan.ne.jp/nature/