Daikonjima Second Lava Tunnel

Designated in 1935: A Tunnel That Matches Those Below Mount Fuji

Daikonjima Second Lava Tunnel (大根島第二熔岩隧道, also known as Ryukeido) is a lava cave on Daikonjima island at Yatsuka-cho, Matsue City, Shimane Prefecture, designated a Natural Monument on 7 June 1935. Matsue City was named its managing body on 26 July of the same year.

The Agency for Cultural Affairs sets out the citation in classical Japanese. The tunnel lies near the centre of the island. Its form runs almost straight from southeast to northwest, extending 81.1 metres, with a single branch tunnel dividing northward from the middle. The floor is nearly level, sloping gently toward the northwest.

The citation then turns to detail. The lava surface of the floor is ropy, with shelf-like ridges along both sides, and short lava stalactites hang from the ceiling. It closes by stating that these features run almost exactly parallel to the lava tunnels at the foot of Mount Fuji.

That closing line carries the designation. A lava tunnel outside Fuji sharing the characteristics of those below it is what made this a Natural Monument.

Why There Is a Lava Cave in Shimane

Daikonjima is a volcanic island in Nakaumi lagoon, about 12 kilometres around. Otsukayama near its centre rises only 42 metres but marks the site of a crater.

A lava tunnel forms when the surface of a lava flow cools and hardens while the still-fluid interior drains away. The surface becomes a shell and the contents empty out. The ropy lava on the floor and the stalactites on the ceiling both record the shape the lava held while it was still hot and moving.

Lava caves designated Natural Monuments in Japan are few, and most cluster at the foot of Mount Fuji in Yamanashi and Shizuoka. Daikonjima holds this one and, separately designated, the First Lava Tunnel, known as Yukido. The first is a Special Natural Monument but is closed because of the risk of rockfall.

That the same island holds two lava tunnels of different origin is worth noting. The first is explained as a gas pocket within a lava flow; the second as the cavity left when the interior of a flow drained away.

A Crater Reported Inside the Cave

One point drew attention long after the designation, and it does not appear in the citation.

A survey by the Vulcanospeleological Society in 2004 reported a lava outflow vent inside the cave, in effect a crater within the tunnel. A dome-shaped space called Kamidamari is understood as a trace of lava swelling and setting as the eruption ended.

This report forms no part of the 1935 reasoning. The Agency's citation goes no further than the form of the tunnel, its floor and its ceiling. The finding is best read as value added after designation, and this article goes no further than that.

Creatures Adapted to the Dark

No light reaches inside, and organisms adapted to that condition have been recorded.

An amphipod with degenerated eyes lives here. A ground beetle is also known from this cave and has not been found elsewhere; the number of individuals recorded is very small.

The temperature inside holds at about 15 degrees Celsius through the year. An environment cut off from outside change supports these animals. Meeting them on a guided visit is a matter of chance, but the damp underfoot and along the walls makes the closed nature of the system easy to feel.

Visiting: Locked, and Guided

The cave is kept locked. Entry requires joining a guided tour, run by the Izumo Geoguide Association.

Scheduled tours run at 2:00 p.m. on Saturdays and Sundays and take about 40 minutes. The fee is 500 yen per person, free for junior high school students and younger, with a maximum of 15 people per tour. Groups and weekday visits can be arranged in advance. Times change with weather and scheduling, so confirm before travelling.

Torches and rubber boots are lent free of charge. The interior is completely dark and the footing is slippery, so wear clothes you do not mind soiling. Children's boots are not always available, so families should bring their own.

From central Matsue it is about 30 minutes by car, crossing the Eshima Ohashi bridge onto the island. The site is at Terazu in Yatsuka-cho.