Daikon Island Lava Tunnel

Designated in 1931: A Tunnel Within a Lava Plateau

Daikon Island Lava Tunnel (大根島の熔岩隧道, Daikonjima no Yogan Zuido, also known as Yukido) is a lava cave at Osoe, Yatsuka-cho, Matsue City, Shimane Prefecture, designated a Natural Monument on 31 July 1931. Matsue City was named managing body on 8 January 1932. It is reported to have been raised to Special Natural Monument status on 29 March 1952.

The Agency for Cultural Affairs writes its citation in classical Japanese and begins with the island itself. Daikonjima sits at the centre of Nakaumi lagoon, about 10 kilometres around, with Otsukayama at its middle rising to 42 metres and the ground falling gently away on every side to a low plateau some 10 metres high. The whole is dark grey porous basalt, which shows the island to be a lava plateau erupted on the lagoon floor. The tunnel formed within that plateau.

The citation's closing line states the reason for designation. The old and new caves were once continuous at the entrance, forming a single lava tunnel in basalt running from southwest to northeast, and unlike the lava tunnels within lava flows at the foot of Mount Fuji, this one shows the new fact that a lava tunnel can form within a lava plateau.

The Opposite Reasoning from the Second Tunnel

The same island holds the separately designated Daikonjima Second Lava Tunnel, known as Ryukeido. Set the two citations side by side and the reasoning runs in opposite directions.

The citation for the second tunnel, written in 1935, lists the form of the cave and the ropy lava of its floor and concludes that these run almost exactly parallel to the lava tunnels below Mount Fuji. Its value lies in being like Fuji.

The citation for this tunnel, written in 1931, concludes that unlike the tunnels within lava flows at the foot of Fuji, it shows that a tunnel can form within a lava plateau. Its value lies in being unlike Fuji.

The standard did not shift in four years. Two caves on one island were each measured against the foot of Fuji, and one was recognised for resemblance, the other for difference. The same kind of landform was valued in two ways according to how it came to be.

The Old Cave and the New: A Ring and a Line

The opening lies near the shore at Osoe on the east of the island, where part of the tunnel roof collapsed. From there the section running southwest in a ring is called the old cave. The entrance sits at the northeast of the ring, and from its western corner a separate passage runs off to the southwest.

The ring is about 100 metres around. The ceiling reaches 2.5 metres at its highest but drops to a single metre at its lowest, where passage is possible only by crawling. Mud lies thick on the floor, and standing water in places reaches the knee.

In the summer of 1926 a second passage was found running in the opposite direction from the old cave and named the new cave. The Agency's text gives the year as the first of Showa, but as that era began on 25 December 1926, the summer in question falls in the fifteenth year of Taisho. The new cave slopes gently northeast in a nearly straight line and can be entered for 62 metres, beyond which mud and water block further progress.

Yet the sound of waves can be heard at its far end, and when a well was dug near the shore 10 metres from that end, a cavity was found at the bottom. From these it was known that the passage connects to the sea. That the cave opens to the water ties in with the understanding of the whole island as a plateau erupted on the lagoon floor.

A Gas-Pocket Origin Reported Later

The citation records the form and position of the tunnel but does not say how it formed. That question was taken up in later work.

A survey by the Fuji Volcanic Cave Research Society, now the Vulcanospeleological Society, in 2003 and 2004 reported that this tunnel did not form by the draining of a lava flow's interior. Instead, volcanic gas within the flow gathered into pockets, and the pockets linked into a tunnel. The opening is thought to have formed when gas pressure broke through a thin roof and the roof collapsed.

This origin contrasts with the second tunnel, explained as the cavity left when a flow's interior drained away. The two caves on the island pair off in their explanations as well as in their citations.

Cave formation by gas accumulation is regarded as unusual among lava caves, but the report forms no part of the 1931 reasoning and is kept separate here as understanding added after designation.

Visiting: Closed to Entry

Yukido is closed because of the risk of rockfall. Viewing is limited to the exterior; the position of the opening and the surrounding ground can be seen, but the interior cannot be entered.

Anyone wanting to experience the inside of a lava tunnel can join a guided tour of the Second Lava Tunnel, Ryukeido, on the same island. It is a separate designation with a different origin and a different reasoning. Reading both citations before entering Ryukeido makes clear why the cave in front of you was judged like Fuji and the closed Yukido unlike it.

The site is at Osoe in Yatsuka-cho, about 30 minutes by car from central Matsue across the Eshima Ohashi bridge. Enquiries go to the Matsue City cultural properties division on 0852-55-5523.