Daikonshima Lava Tunnels: Ancient Volcanic Caves Beneath a Tranquil Island

Hidden beneath the gentle farmlands of Daikonshima — a small volcanic island floating in the brackish waters of Lake Nakaumi in Shimane Prefecture — lies a remarkable underground world. The Daikonshima Lava Tunnels are a pair of nationally designated natural monument caves formed approximately 200,000 years ago by volcanic eruptions. The primary tunnel, known locally as Yūkidō (幽鬼洞, "Ghost Cave"), holds the prestigious distinction of being one of Japan's Special Natural Monuments, while the nearby second tunnel, Ryūkeidō (竜渓洞, "Dragon Ravine Cave"), is designated as a Natural Monument. Together, they offer a rare glimpse into the geological forces that shaped this extraordinary island.

What Are the Daikonshima Lava Tunnels?

Daikonshima and the neighboring island of Eshima were originally formed by a single volcanic system that erupted over a period of roughly 30,000 years, from approximately 220,000 to 190,000 years ago. The islands are composed entirely of basalt — a dark, fine-grained volcanic rock with very low viscosity when molten. Unlike the dramatic cone-shaped volcanoes familiar to most visitors, Daikonshima is a shield volcano with a gentle, plateau-like profile. Its highest point, Mount Ōtsuka, rises only 42 meters above sea level.

The lava tunnels were created within the thick basalt layers that cover the island to a depth of approximately 80 meters. Rather than forming from a single massive eruption, these layers were built up gradually as thin flows of lava seeped from numerous small vents across the island's surface.

A Unique Formation Process

For many years, scientists assumed the Daikonshima lava tunnels were formed in the same way as the well-known lava tubes near Mount Fuji — where the surface of a flowing lava stream cools and solidifies while the molten interior drains away, leaving a hollow tube behind. However, detailed surveys conducted in 2003 and 2004 by the Fuji Volcanic Cave Research Society (now the Japan Speleological Society for Volcanic Caves) revealed a fundamentally different origin.

The Daikonshima tunnels are now understood to be "gas-pocket cavities." As the basalt lava flowed and began to cool, volcanic gases trapped within the molten rock bubbled together to form pockets. As these gas-filled spaces expanded and migrated through the still-fluid lava, they merged with one another, creating interconnected cavities that eventually formed the tunnel-like passages we see today. The entrance to the primary tunnel is believed to have formed when a large gas pocket ruptured through the thin crust of solidified lava at the surface.

This gas-accumulation formation mechanism is exceptionally rare among the world's lava caves, making the Daikonshima tunnels a site of outstanding geological significance.

Why Were They Designated as Natural Monuments?

Japan has 13 nationally designated lava cave natural monuments, and 11 of these are clustered around the base of Mount Fuji in Yamanashi and Shizuoka prefectures. The remaining two are both located on Daikonshima, making them geologically exceptional outliers far from Japan's more famous volcanic regions.

The primary lava tunnel (Yūkidō) was first designated as a Natural Monument on July 31, 1931, and was elevated to Special Natural Monument status on March 29, 1952 — one of the highest levels of cultural property protection in Japan. This upgrade recognized the tunnel's unique loop-shaped passage system, its scientific importance in understanding basaltic volcanic processes, and the presence of rare geological formations including lava stalactites and globular concretions (gyokutekiseki).

The second tunnel (Ryūkeidō) was discovered accidentally during road construction in 1933 and designated as a Natural Monument on June 7, 1935. Subsequent research in 2004 revealed that this cave contains an internal lava vent — essentially a cave-within-a-cave fire vent — a formation type with no other known examples in Japan.

Exploring Yūkidō: The Ghost Cave

Yūkidō consists of two interconnected sections: the "Old Tunnel" (kyūdō) and the "New Tunnel" (shindō), with a combined length of approximately 206.6 meters. The Old Tunnel forms a loop-shaped passage roughly 100 meters in circumference, while the New Tunnel extends in a straight line for about 70 meters toward the northeast.

Inside the Old Tunnel, visitors once encountered named features such as "Senuri" (narrow passages where explorers had to squeeze through on their backs), "Senjōjiki" (a broad chamber called "The Thousand-Tatami Room"), "Oni no Ido" (The Demon's Well), and "Oni no Nedoko" (The Demon's Bed). The cave walls are decorated with lava stalactites and globular concretions formed from mineral deposits.

The New Tunnel was discovered in 1925 and extends toward the coast. Deep within its passage, observers have reported hearing the sound of ocean waves, and a well dug near the coast confirmed that the tunnel connects to the sea. Remarkably, fish with degenerated eyes — a hallmark of true cave-adapted species — have been found within the cave system.

Please note that Yūkidō is currently closed to the public due to the risk of rockfall. Visitors can still view the cave entrance and read interpretive signage at the site.

Exploring Ryūkeidō: The Dragon Ravine Cave

While the Special Natural Monument Yūkidō remains off-limits, the nearby Ryūkeidō offers an unforgettable underground adventure. This approximately 80-meter-long cave winds its way beneath the island's farmland and can be explored with the guidance of certified geoguides from the Izumo-kuni Geoguide Association.

The cave interior reveals vivid traces of ancient volcanic activity. Lava flow patterns are preserved on the walls in both linear streaks and dramatic spiral formations. The cave also features what researchers have identified as an internal lava vent — a point where molten rock once erupted within the cave itself. This feature is considered extremely rare and scientifically valuable, with no other confirmed examples in Japan.

Ryūkeidō is also home to a unique ecosystem of cave-dwelling organisms, including species of shrimp and other creatures whose eyes have degenerated over thousands of generations in total darkness. Some of these species are found nowhere else on Earth.

Guided tours last approximately 40 minutes. Rubber boots and flashlights are provided, as the cave interior is completely dark and the ground can be slippery. The experience of descending into the earth and walking through a passage shaped by ancient volcanic forces is truly one-of-a-kind.

Daikonshima and the Shimane Peninsula Geopark

In December 2017, Daikonshima was certified as part of the Shimane Peninsula and Shinjiko Nakaumi Estuary Geopark, a Japanese Geopark that showcases the region's remarkable geological heritage. On Daikonshima, designated geosites include the lava tunnels, the scoria hill of Mount Ōtsuka, and the natural spring water parks fed by underground volcanic aquifers.

Mount Ōtsuka itself is a scoria cone — a small hill built from fragments of volcanic rock (scoria) ejected during ancient eruptions. It is sometimes called one of the lowest volcanoes in Japan. From its modest summit, visitors can enjoy panoramic views across the island and Lake Nakaumi.

Nearby Attractions

Daikonshima is a compact island that can be circled by car in about 30 minutes, yet it offers a surprising variety of experiences beyond its underground wonders.

Yuushien Garden: This celebrated Japanese stroll garden spans over 40,000 square meters and is renowned for its year-round peony displays. During Golden Week in early May, over 30,000 peony blossoms are floated on the garden's central pond in a breathtaking spectacle. The garden also features a Korean ginseng museum, seasonal illumination events, and tea houses overlooking the beautifully landscaped grounds.

Eshima Ōhashi Bridge: Connecting Daikonshima to Sakaiminato in Tottori Prefecture, this dramatic bridge has become an internet sensation due to photographs that make it appear impossibly steep. It is the third-largest rigid-frame bridge in the world and offers stunning views of Lake Nakaumi.

Peony Farms and Nurseries: Daikonshima produces approximately 1.8 million peony plants annually, making it Japan's largest peony-growing region. Multiple nurseries welcome visitors, especially during the Peony Festival in late April and early May.

Unshu Ginseng (Korean Ginseng): The island has cultivated high-quality Korean ginseng for over 200 years. Known as "Unshū Ninjin," this ginseng benefits from the mineral-rich volcanic soil and is considered among the finest in the world.

Getting There

Daikonshima is connected to the mainland by bridges and levee roads, making it easily accessible by car. From central Matsue, the island is approximately 20 minutes by car. From Sakaiminato (home of the Mizuki Shigeru Road and the GeGeGe no Kitarō Museum), the island is about 15 minutes via the Eshima Ōhashi Bridge. Community buses also connect the island to both cities, though a rental car is recommended for the most flexible exploration of the lava tunnel sites and surrounding attractions.