Pyroclastic Rock and the Fukuura Tunnels
At this site you can see tunnels of various sizes that were dug into the steep wave-eroded cliff in the middle of a bay in Dōgo Island.
These tunnels were dug and expanded at different times throughout history. The smallest tunnels were dug using hand tools during a time when transport in Oki was limited to boat and on foots. This allowed people to walk safely along the sea-eroded side of the mountain. A larger tunnel was later dug beside them with tools and dynamite. This tunnel was large enough to fit a horse and cart. Later on, this tunnel was widened using heavy machinery in order to be wide enough to fit a school bus, signifying the progression of transport to automobiles.
These tunnels have preserved the stages of development of civil engineering and transport over time. For this reason, they were designated as Civil Works Heritage of Japan.
The tunnels are also valuable in another sense. The cliff they were dug into is composed of pyroclastic rock which erupted six million years ago. Pyroclastic rock is very soft, which is why the tunnels could initially be dug by hand. Today you can have the unique experience of walking through tunnels dug at different times throughout history, all the while observing the interior structure of pyroclastic flow.