Daikonshima Second Lava Tunnel (Ryukeido): A Rare Underground Journey into Japan's Volcanic Past

Hidden beneath the tranquil surface of Daikonshima Island in Shimane Prefecture lies a remarkable natural wonder — the Daikonshima Second Lava Tunnel, locally known as Ryukeido (Dragon Valley Cave). Designated as a National Natural Monument of Japan in 1935, this lava tube offers visitors a rare chance to step inside a 190,000-year-old volcanic formation and witness geological features found almost nowhere else on Earth.

Daikonshima sits in the middle of Lake Nakaumi, a brackish lagoon straddling the border of Shimane and Tottori prefectures. Despite its flat, unassuming landscape, the island is in fact a volcanic island — born from eruptions that took place approximately 200,000 years ago. The summit of Otsukayama, at just 42 meters above sea level, holds the distinction of being one of the lowest volcanoes in all of Japan. The Ryukeido lava tunnel is living proof of that fiery origin.

How the Lava Tunnel Was Formed

The Daikonshima Second Lava Tunnel was created during the Late Pleistocene period, roughly 190,000 years ago, when basaltic lava erupted from small vents on the island. As the surface of the lava flow cooled and hardened, the still-molten interior continued to flow, eventually draining away and leaving behind a hollow tube approximately 81 meters in length.

What makes this tunnel scientifically extraordinary is the discovery, during a detailed survey in 2004 by the Volcanic Cave Society (formerly the Fuji Volcanic Cave Research Association), that the tunnel contains an intra-cave lava vent — essentially a volcanic crater inside the cave itself. This is an extremely rare geological feature with no other known example in Japan. The formation, called "Kamidamari" (divine pool), is a dome-shaped cavity believed to mark the point where lava last erupted and then solidified as volcanic activity ceased.

Unlike the nearby First Lava Tunnel (Yukido), which was formed by gas pockets trapped within stagnant lava, the Second Lava Tunnel was created by the internal flow of lava beneath an already-hardened crust. This distinction makes the two tunnels complementary examples of different volcanic processes occurring on the same small island.

Why It Was Designated a Natural Monument

The Daikonshima Second Lava Tunnel was designated a National Natural Monument on June 7, 1935. The official designation noted the tunnel's nearly straight formation extending 81.1 meters from south to northwest, with a branch tunnel diverging from the central section. The cave floor displays distinctive ropy (pahoehoe-type) lava textures, with shelf-like ridges along both walls and short lava stalactites hanging from the ceiling — features comparable to those found in the famous lava tunnels at the base of Mount Fuji.

In Japan, only 13 lava caves have been designated as Natural Monuments. Of these, 11 are located near Mount Fuji in Yamanashi and Shizuoka prefectures. The remaining two are both on Daikonshima Island, making this tiny volcanic island an exceptionally significant site for volcanological research. The First Lava Tunnel holds the even higher distinction of Special Natural Monument status, though it is currently closed to the public due to the risk of collapse.

Highlights and What to See

Descending into Ryukeido is a true adventure. The entrance leads underground to a pitch-dark world where the temperature remains at a constant 15°C year-round — refreshingly cool in summer and surprisingly warm in winter. Visitors are equipped with flashlights and rubber boots for the approximately 40-minute guided exploration.

Inside, the cave walls vividly preserve the traces of ancient lava flows in both linear and swirling patterns. The ceiling features small lava stalactites — droplets of molten rock that solidified in mid-drip thousands of years ago. The dome-shaped Kamidamari chamber, believed to be an intra-cave volcanic vent, is a highlight that conveys the raw power of the island's volcanic origins.

Perhaps the most fascinating aspect of Ryukeido is its unique cave ecosystem. The perpetual darkness has given rise to highly specialized organisms, including eyeless amphipod crustaceans that have adapted to life without light. The cave is also home to a species of ground beetle (genus Trechiama) found nowhere else in the world — with only nine individuals ever recorded. Encountering these rare creatures during a visit is a matter of extraordinary luck, but their very existence underscores the cave's immense scientific value.

Visiting Information and Guided Tours

The Daikonshima Second Lava Tunnel is normally kept locked and can only be entered with a guided tour. Tours are conducted by members of the Izumo-no-Kuni Geoguide Association, who provide expert commentary on the cave's geology, biology, and cultural significance.

Scheduled guided tours are available on Saturdays and Sundays at 2:00 PM (subject to weather conditions and schedule changes). Private group tours can also be arranged by contacting the geoguide association in advance. The guide fee is 500 yen per person, with free admission for junior high school students and younger. Each tour accommodates up to 15 participants.

Flashlights and rubber boots are provided free of charge. The cave interior is completely dark and the floor can be slippery, so visitors should wear clothing that can get slightly dirty. Children's rubber boots may not be available, so families are advised to bring their own.

Daikonshima Island and Surrounding Attractions

Daikonshima is a compact island — just 3 kilometers east to west and 2 kilometers north to south — that can be circled by car in about 30 minutes. Despite its small size, the island offers a surprising variety of attractions beyond the lava tunnels.

Access

By car, Daikonshima is approximately 20 minutes from central Matsue via Route 431, crossing the Nakaumi Ohashi Bridge. From Sakaiminato, the island is about 15 minutes by car via the Eshima Ohashi Bridge. Public bus service is available from Matsue Station (Matsue City Bus), with the nearest stop at Yushien-iriguchi. Visitors arriving by bus should note that the lava tunnel is a short walk from the main road, and having a rental car or taxi is recommended for the most convenient access.