Onogoshi Drainage Tunnel

What Is the Onogoshi Drainage Tunnel?

The Onogoshi Drainage Tunnel (大野越排水溝, Ōnogoshi Haisuikō) is a reinforced concrete drainage tunnel built in 1934 under the Ōno forest in Higashinakasonezoe, Hirara, Miyakojima City, Okinawa Prefecture, and it was registered as a Registered Tangible Cultural Property of Japan on July 31, 2007.

The Japanese name literally means "drainage ditch," which undersells it. The Onogoshi structure is a bored channel that runs 640 meters underground, and Miyakojima City itself calls it the Onogoshi Drainage Tunnel in English. The city owns it, and its registration number is 47-0045.

Miyako Island is a low, flat limestone island in the Sakishima chain southwest of Okinawa's main island. The Agency for Cultural Affairs places the tunnel in the northeast of the city and describes it as basic infrastructure for bringing land under cultivation. What a visitor can see above ground is a tunnel mouth in the woods; nearly everything else lies out of sight.

Why the Onogoshi Tunnel Was Dug

According to Miyakojima City, the area around the Onogoshi Drainage Tunnel was low, marshy ground that drained badly and stayed wet all year. To make it usable for farming, the national government drew up the Ōnoyama land readjustment plan, and the tunnel was built as one of its projects: water that pooled in the wetland would be led away underground, leaving fields that could be worked.

The agency's commentary adds a second outcome. As a drain for the marsh, the tunnel proved effective against the mosquitoes that carry malaria. Standing water is where those mosquitoes lay their eggs and where the larvae grow, so drying the ground also took away their breeding places. A farming project ended up doing duty as public health work.

How the Onogoshi Drainage Tunnel Is Built

In cross-section the Onogoshi Drainage Tunnel has a segmental arch, a roof formed by a shallow slice of a circle rather than a full half-round (the city simply calls it arch-shaped). Miyakojima City gives the opening as about 1.7 meters high and 1.8 meters wide, and that profile continues for the full 640 meters.

The agency singles it out as one of the earliest reinforced concrete structures on Miyako Island. The city goes further, noting that the tunnel is large for an early Shōwa-era tunnel and reflects both the engineering standard of its day and the circumstances that produced it.

The east portal is where the builders spent effort on appearance. Although it is concrete, it is finished to read as a masonry arch, complete with a shape modeled on a keystone at the crown. A purely functional drain was given a doorway that looks like dressed stonework.

The tunnel was registered under the criterion of structures that contribute to the historic landscape of the country. More than 90 years after it was built it still carries water, and the city describes its condition as relatively good.

Seeing the Onogoshi Drainage Tunnel

The Onogoshi Drainage Tunnel is a working drain owned by Miyakojima City, not a visitor attraction. The city's heritage page gives no route to the portals and no viewing arrangements, and there are no opening hours or fees. The inside is a waterway, so do not plan on entering the tunnel or climbing down into the channel. There is no official guidance on photography.

The tunnel lies inside the Ōno forest (Ōno Sanrin). The coordinates in the national heritage database point to a spot about 1 kilometer northeast of the Miyakojima City Tropical Botanical Garden, roughly 4 kilometers northeast of Miyako Airport in a straight line. The map on the city's page shows the general location, but paths in the forest can be hard to follow and the ground may be rough. Before going, ask the Lifelong Learning Promotion Division of the Miyakojima City Board of Education (phone 0980-72-3764, Japanese) where the portal is and whether it can be approached.