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Located along Prefectural Road 300 (Ujishita-hara Line) in the mountainous region of Takahashi City, Okayama Prefecture, the "Hayama Dai-ni Tunnel" is an extraordinary structure where natural formation meets man-made engineering. The most striking feature is that the tunnel utilizes a natural limestone cave. Standing at the northwest entrance, you are greeted by towering limestone walls that create the sensation of entering a massive natural cavern. As one of the few "unlined" (raw-cut) tunnels in Okayama, it offers a rare opportunity to observe both historical engineering and the raw beauty of nature simultaneously.
The history of this tunnel dates back to the Taisho era, with construction starting in 1919 and completion in 1921. At the time, it played a vital role in transporting goods. Historically, the route connecting Biba and Seiya passed through ridge lines, transporting essential goods like ochre (bengala), copper, iron, and seafood from Kasaoka. To accommodate the increasing traffic of horse-drawn carriages and handcarts, this route along the Kikawa River was excavated to provide a more efficient transport path. The tunnel was designed with a width of 3 meters to allow carts and horses to pass through.
There are several points of interest at the Hayama Dai-ni Tunnel. First is the northwest entrance, where the vertical limestone walls of the karst topography create a dramatic, cave-like atmosphere. Second is the "unlined" interior structure. Spanning 32 meters in length and 3 meters in width, the interior lacks modern concrete lining, leaving the raw, rugged rock face exposed. Additionally, the steep limestone walls near the northwest entrance are so dramatic that the area is occasionally used as a site for rock climbing enthusiasts.
As this is part of a public road (Prefectural Road 300), it is accessible at any time of year. Visiting during daylight hours is ideal for appreciating the intricate textures of the limestone walls and the details of the karst landscape. It is highly recommended to enjoy this view alongside the surrounding Hayama Valley scenery. However, please note that the interior of the tunnel and nearby natural crevices (such as the Seiryu Cave) lack artificial lighting, so visibility will be very low during nighttime or in dark weather conditions.
The primary experience is enjoying the scenery during a drive or using the site as a photo spot. The entrance offers a stunning backdrop for photography, capturing the raw beauty of the natural rock formations. For outdoor enthusiasts, the nearby cliffs are known as a training ground for rock climbing. While exploring, please be mindful that the tunnel width is limited, so drivers should proceed with caution.
When visiting the Hayama Dai-ni Tunnel, consider combining your trip with a drive through the scenic Hayama Valley or toward the Tenryu Bridge to the west. Nearby, you can find the "Kata-domon" (a natural rock gate where a rock overhang covers the path) and the "Hayama Dai-ichi Tunnel," which has a more modern concrete entrance. These sites together offer a continuous and breathtaking display of massive limestone cliffs. The area is approximately 30km from the Chugoku Expressway's Niimi IC, making it an easy addition to a regional driving itinerary.
Since this is a public road, there are no entrance fees or reservations required. However, as it is a natural environment, comfortable walking shoes and clothing suitable for outdoor exploration are recommended. Be cautious of damp surfaces or low visibility near the rock walls depending on the weather. For those driving through, please maintain a slow and safe speed due to the narrow passage.