Ryugu Cave: A Sacred Lava Tunnel in Aokigahara at the Foot of Mt. Fuji
Hidden deep within the primeval Aokigahara forest at the northern foot of Mt. Fuji lies one of Japan's most atmospheric sacred sites: Ryugu Cave (龍宮洞穴, Ryugu-Dōketsu), literally "the Dragon Palace Cavern." This 60-meter-long lava tube, designated a Natural Monument by the Japanese government in 1929, weaves together volcanic geology, Shinto mythology, and centuries of Mt. Fuji worship into a single quiet hollow in the forest floor.
Unlike the polished show caves nearby, Ryugu Cave is not a tourist attraction in the conventional sense. It is a place of pilgrimage—a place where, for over a thousand years, people have come to pray for rain, for water, and for the blessing of the dragon believed to dwell beneath the moss-covered lava.
A Cave Born from a Cataclysmic Eruption
To understand Ryugu Cave, one must first understand the landscape that cradles it. Aokigahara—the "Sea of Trees"—is a vast forest of black pine, hemlock, and cypress that grew up over an immense flow of cooled lava. That lava was deposited in the year 864, when Mt. Fuji erupted in what is known as the Jōgan Eruption, one of the most violent in the volcano's recorded history.
Before the eruption, the area was occupied by a single large lake called Se-no-Umi (剗海). The torrent of lava split this lake into two surviving bodies of water—Lake Saiko and Lake Shōji—and buried the rest beneath a hardened stone shell on which the forest later took root.
Ryugu Cave is a lava tube created by this same flow. As the surface of the molten lava cooled and crusted over, the still-liquid lava beneath continued to drain away, leaving behind hollow tunnels and chambers. The interior remains so cool year-round that ice can sometimes be seen on the cave floor even in midsummer.
The Water Goddess of the Dragon Palace
At the entrance to the cave stands a small stone shrine (hokora). It enshrines Toyotama-hime no Mikoto (豊玉姫命), the daughter of the sea god Watatsumi and a goddess of water. In the ancient Japanese chronicles, Toyotama-hime is described as transforming into a dragon (or in another version, a crocodile) at the moment of childbirth—an image that resonates powerfully with the cave's Japanese name, which means "Dragon Palace."
According to local records, in the year 913 (Engi 13), a severe famine drove villagers to invoke Toyotama-hime here, praying for favorable weather and abundant harvests. This shrine, an inner sanctuary of Senoumi Shrine, is said to have been founded in that act of supplication. Centuries later, during the Edo period, the deity of Mt. Fuji—Asama no Ōkami—was also enshrined alongside her.
The cave became renowned across the Kantō region as a place where water borrowed from beside the shrine and offered up in prayer would unfailingly bring rain. It earned its place as one of the eight sacred sites visited by Fuji-kō pilgrims on their Hakkai-meguri circuit of holy waters.
Why the Cave Was Designated a Natural Monument
Ryugu Cave was designated a National Natural Monument on December 17, 1929. The reasoning is twofold and reflects how Japan's heritage system can honor both science and culture in a single site.
Geologically, the cave is a textbook example of a lava tube formed under the special conditions of Mt. Fuji's basaltic eruptions. The official designation notes that the proliferation of such caves around Mt. Fuji—of which Ryugu is one of many—is rare on a global scale. The dense concentration of lava tubes, gas-pocket caves, ice caves, and tree-mold formations across the Aokigahara plateau makes the area scientifically invaluable for understanding volcanic processes.
Culturally and historically, the cave has functioned for over a millennium as a religious site connected to Mt. Fuji worship. It is recognized today not only as a Natural Monument in its own right, but also as part of the broader cultural landscape inscribed on the UNESCO World Heritage List as "Fujisan, Sacred Place and Source of Artistic Inspiration."
What to See and Experience
The approach itself is part of the experience. A short footpath leaves the road and threads through dim, moss-cushioned Aokigahara forest. Within minutes the temperature drops, the sounds of the outside world fall away, and you arrive at a depression in the ground where the cave entrance opens like a wound in the lava.
Visitors should be aware that entry into the cave itself is prohibited. Ceiling collapses in recent decades have left fallen rocks across the floor, and ongoing risk of rockfall makes the interior dangerous. The route down to the small shrine just inside the entrance, however, is generally accessible, and this is where most pilgrims and visitors pause to pay their respects.
Standing at the lip of the cave, even in high summer, you may feel a sudden chill—cold air seeping out from somewhere deep below. On humid days, the temperature differential causes wisps of mist to coil out of the darkness. It is easy to understand how, for generations, this place was seen as the breath of the dragon.
Every year on August 2, the Ryugu Festival (Ryugu-sai) is held here. It is a quiet, locally rooted ceremony that continues the centuries-old tradition of honoring the water deity of the cave.
Nearby Attractions in the Sea of Trees
Ryugu Cave sits at the heart of one of Japan's most extraordinary volcanic landscapes, and pairs beautifully with several nearby sights.
- Fugaku Wind Cave (Fugaku Fūketsu) — a long lava tube open to the public, where you can walk into the volcanic darkness and see ice columns that persist into summer.
- Narusawa Ice Cave (Narusawa Hyōketsu) — a permanent ice cave used in earlier centuries to store silkworm eggs for the silk industry.
- Saiko Bat Cave (Saiko Kōmori-ana) — the longest lava tube in the area, home to several species of cave-dwelling bats.
- Lake Saiko — one of the Fuji Five Lakes, formed when the Jōgan lava flow split the ancient Se-no-Umi in two.
- Aokigahara Forest Walking Trails — well-maintained paths offer a safe and richly atmospheric way to experience the Sea of Trees on foot.
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Q＆A
- Can I go inside Ryugu Cave?
- Entry into the main body of the cave is prohibited due to severe ceiling collapse and ongoing risk of falling rocks. Visitors can, however, descend to the cave entrance area where the small shrine to Toyotama-hime is located. Please observe local guidance and proceed at your own responsibility.
- How do I get to Ryugu Cave from Tokyo?
- Take a JR or highway bus to Kawaguchiko Station. From there, board the Saiko Retro Bus (Green Line) and alight at "Ryugu-Dōketsu Iriguchi" (Ryugu Cave Entrance) bus stop. The cave is a short walk through the forest from there. Alternatively, the Blue Line bus to Fugaku Wind Cave puts you within a 20-minute walk.
- Why is the cave called "Dragon Palace"?
- The name reflects the cave's association with water and dragons. The water goddess Toyotama-hime, enshrined here, is said in ancient Japanese myth to have transformed into a dragon. Combined with the cool, mist-breathing nature of the cave and the local belief that a dragon dwells beneath the forest, the name "Ryugu" (Dragon Palace) has been used for centuries.
- When was Ryugu Cave designated a Natural Monument, and why?
- It was designated on December 17, 1929. The designation recognizes the cave as an outstanding example of a lava tube formed by Mt. Fuji's eruptions, and notes that the dense concentration of such caves around Mt. Fuji is rare on a global scale. The site is also valued for its long history as a place of religious pilgrimage.
- Is there an admission fee?
- No. Ryugu Cave is a free and openly accessible site of natural and religious heritage. However, please remember that this is a sacred place and a fragile natural monument—please respect local rules, take all rubbish away with you, and do not disturb the shrine.
Basic Information
| Name | Ryugu Cave (龍宮洞穴, Ryugu-Dōketsu) |
|---|---|
| Designation | National Natural Monument (Tennen Kinenbutsu) |
| Date Designated | December 17, 1929 (Shōwa 4) |
| Location | Aokigahara, Fujikawaguchiko Town, Minamitsuru District, Yamanashi Prefecture |
| Type | Lava tube (volcanic cave) |
| Total Length | Approximately 60 meters |
| Origin | Formed by the lava flow of the Jōgan Eruption of Mt. Fuji (864 CE) |
| Enshrined Deity | Toyotama-hime no Mikoto (water goddess); also Asama no Ōkami (added in the Edo period) |
| Affiliated Shrine | Senoumi Shrine (inner sanctuary) |
| Religious Significance | Fifth of the eight sacred waters (Hakkai-meguri) of the Fuji-kō pilgrimage tradition |
| Annual Festival | Ryugu-sai, held on August 2 each year |
| Managing Body | Fujikawaguchiko Town (since June 4, 1996) |
| Access | About 5 minutes on foot from "Ryugu-Dōketsu Iriguchi" bus stop on the Saiko Retro Bus (Green Line) from Kawaguchiko Station |
| Admission | Free; cave interior closed for safety reasons |
References
- 龍宮洞穴 - 文化遺産オンライン (Cultural Heritage Online, NII)
- https://bunka.nii.ac.jp/heritages/detail/190712
- 国指定文化財等データベース - 龍宮洞穴 (National Cultural Properties Database, Agency for Cultural Affairs)
- https://kunishitei.bunka.go.jp/heritage/detail/401/2578
- 竜宮洞穴／富士の国やまなし観光ネット (Yamanashi Prefecture Official Tourism Site)
- https://www.yamanashi-kankou.jp/kankou/spot/p1_4357.html
- 剗海神社／山梨の歴史を旅するサイト (Senoumi Shrine, Yamanashi History Travel Site)
- https://www.yamanashi-kankou.jp/rekitabi/jisha/spot/196.html
- 山梨の文化財リスト（天然記念物）- 山梨県
- https://www.pref.yamanashi.jp/bunka/bunkazaihogo/kinenbutsu.html
Last verified: 2026.05.14