Hello, this is Takezawa.
Today, let’s continue our exploration of Mount Fuji!
Our theme is “Lava Caves,” and as always, our reference material is the “Mount Fuji Certification” guide.
Note: Some details, such as dates, might differ from current theories, but we’ll primarily refer to this book as our source.
What Are Lava Caves?
As the name suggests, they are “caves formed from lava.” But let’s delve a bit deeper to truly understand them.
Formation of Lava Caves
There are over 100 lava caves at the base of Mount Fuji, ranging from just a few meters to over 2,000 meters in length. Some are on private property and not open to the public, while others are popular tourist spots or even part of World Heritage sites. These caves vary widely in size and condition.
So, how are these lava caves formed?
The type of lava plays a crucial role. Mount Fuji is covered with basaltic lava, which is highly fluid. As this lava flows straight down, the outer layer cools and solidifies upon contact with air or the ground, while the interior remains molten and continues to flow. Eventually, the internal lava breaks through the hardened surface, creating hollow spaces.
Alternatively, gases or steam trapped within the lava can increase internal pressure, forming large cavities inside the lava.
It’s helpful to visualize this process. For instance, when baking mochi (rice cakes), they puff up. This happens because the moisture inside turns into steam, pushing against the surface—a similar concept to internal pressure in lava.
Lava Caves as Objects of Legend and Worship
Many lava caves, with their eerie appearances, have become subjects of legends and worship. Notable examples include “Hitoana” (a component of the World Heritage site), “Ryugu Cave” in the Aokigahara Sea of Trees, and “Suyama Gotenai,” which enshrines Konohanasakuya-hime. These caves have contributed to Mount Fuji’s designation as a World Heritage site under the criterion of “a sacred place and source of artistic inspiration.”
Now, let’s highlight a few lava caves around the base of Mount Fuji.
Hitoana
Located in Fujinomiya City, Shizuoka Prefecture, this lava cave is part of the “Hitoana Fuji-ko Iseki,” a World Heritage component. Approximately 83 meters in length, it’s believed to be the cave where Hasegawa Kakugyo, the founder of Fuji-ko, practiced asceticism. In the past, it was considered a sacred site for Fuji-ko, attracting many practitioners for worship and training.
Currently, entry into Hitoana is prohibited. It’s a secluded spot, but since its designation as a World Heritage component, facilities like restrooms and parking lots have been well-maintained. About 20 years ago, it was known among local youths as a haunted spot.
Saiko Bat Cave
Managed by Fujikawaguchiko Town, this lava cave is located within the Aokigahara Sea of Trees. With a total length exceeding 350 meters, it’s the largest accessible cave at the base of Mount Fuji. As the name suggests, bats inhabit this cave. In the past, thousands to tens of thousands of bats lived here, but due to lack of management, irresponsible tourists, and surrounding development, their numbers drastically decreased, even reaching zero at one point. Currently, through measures like restricting entry times (only during the day), seasonal closures (closed in winter), and designated protection areas within the cave, the bat population has recovered to about 700–1,000.
Narusawa Ice Cave
Also situated within the Aokigahara Sea of Trees, this lava cave is managed and operated by a private company as a popular tourist spot. Especially during the summer, visitors often form lines to enter the cave. The internal temperature averages around 3°C, and it’s covered in ice year-round, hence the name “Ice Cave.” It’s like a natural refrigerator. The cave features a vertically deep structure, reaching about 21 meters underground. Throughout the cave, passages are very narrow and low. Be cautious to avoid bumping your head.
Additionally, the same company operates the “Fugaku Wind Cave,” located about a 30-minute walk away within the Sea of Trees. This cave has a horizontal structure. Similarly, be mindful of low ceilings to avoid injuries.
Other notable caves include:
- Mitsuikeana: A privately-owned cave with a total length of 2,139 meters, currently inaccessible to the public.
- Fuji Wind Cave: Located north of the Aokigahara Sea of Trees, it’s the largest ice-holding cave among those at the northern base of Mount Fuji. Entry requires permission.
- Dainichiana: Named because the Dainichi Nyorai was enshrined at its entrance.
- Babaana: A cave reminiscent of the legendary “Obasuteyama.”
Lava caves are scattered throughout various parts of the Mount Fuji foothills.
Lava Tree Molds
Holes formed when lava engulfs trees, known as “lava tree molds,” are also found around the base of Mount Fuji, especially in the Aokigahara Sea of Trees.
Lava tree molds are cylindrical cavities formed when trees are engulfed by lava, the heat carbonizes and removes the wood, leaving hollow spaces. These vary in size, and the quantity and quality of lava caves around Mount Fuji are considered among the best in the world.
Representative examples include:
Funatsu Tainai Jukei
Located in Funatsu, Fujikawaguchiko Town, this is one of the components of the World Heritage Site, Mount Fuji. The term “Tainai Jukei” (literally “womb tree shape”) is derived from its resemblance to the human womb and was once a subject of religious veneration. This formation is known as a “yoga” (lying down) type, formed by the fusion of at least five trees, extending approximately 68 meters in length. It is situated within the Mutou Sengen Shrine, where visitors can pay an admission fee to experience a “womb tour” inside the cave.
Yoshida Tainai Jukei
Located in Fujiyoshida City, this is also a component of the World Heritage Site, Mount Fuji. Similar to the Funatsu Tainai Jukei, it was formed by multiple trees and has been revered by followers of the Fuji faith since ancient times. Additionally, there are over 60 lava tree molds in the surrounding area, collectively designated as a National Natural Monument under the name Yoshida Tainai Jukei Group. Please note that the interior is not open to the public.
Other notable lava tree molds around the base of Mount Fuji include the “Narusawa Lava Tree Molds” in Narusawa Village at the edge of the Aokigahara Sea of Trees, and the “Inno Lava Tunnel” within the Gotemba City Inno Otainai Seikoen. These formations can be observed in various locations around the foot of Mount Fuji, so be sure to look for them during your trekking adventures.
That’s all. Thank you very much.
