Oinuana Cave is a limestone cave located in Ueno Village, Tano District, Gunma Prefecture. Designated as a National Natural Monument of Japan, it is regarded alongside Fudō Cave, also in Ueno Village, as one of Gunma Prefecture’s two major limestone caves.
The cave was discovered by four local young men in 1929 and has long been recognized for its scientific and geological importance as one of the notable limestone caves in the Tano and Chichibu regions.
Today, the entrance is securely locked to protect the cave environment, and visitors are not permitted to enter. Nevertheless, its history, legends, and remarkable natural formations continue to attract interest.
The name Oinuana is believed to be associated with an ancient local belief in yamainu, or “mountain dogs.” Even before the limestone cave itself was discovered, there was said to be a small rock chamber here, roughly the size of four tatami mats, where mountain dogs were believed to live and raise their young.
According to local tradition, mountain dogs sometimes came close to human settlements during their breeding season and attacked livestock. To protect their animals, villagers offered sekihan (red rice) and mochi rice cakes when the animals were believed to be giving birth.
It was said that livestock escaped harm in years when offerings were made, while attacks occurred in years when they were not. The tradition eventually became an annual ritual known as Otakiage.
Because the cave was regarded as the dwelling place of the revered “Oinu-sama,” or “Honored Dog,” it came to be known as Oinuana, meaning “the mountain dog’s cave.”
The limestone cave was discovered on August 2, 1929, by four local young men who were around 17 years old. Their exploration was inspired by a comment from an elementary school student who had heard what sounded like water dripping somewhere in the mountains.
The young men wondered whether there might be another cave similar to Fudō Cave and began searching the area. Near the suspected cave entrance, they found a small opening large enough for a small animal to pass through.
They cleared away fallen leaves and soil and dug through the clay-like earth until they created a narrow opening large enough for a person to crawl through. When they ventured inside, they discovered a large underground chamber beyond the narrow entrance.
With no proper equipment available, they used dried bamboo as torches and rope as a climbing line. Their exploration was therefore limited that day. The following day, approximately 50 villagers gathered to conduct a more extensive investigation, revealing the remarkable extent of the cave system.
Oinuana Cave was formed when groundwater gradually eroded limestone belonging to the Chichibu Paleozoic strata. From the west-facing entrance, the passage becomes steep approximately 10 meters inside, leading into a four-level vertical cave system descending deep underground.
The cave extends approximately 300 to 450 meters into the mountain and divides into two passages at its deepest point.
The interior is traditionally divided into six sections, each given a name inspired by Japanese mythology and legends. From the entrance toward the depths, these include:
These evocative names give the underground spaces an atmosphere reminiscent of the mythical world of ancient Japan, making Oinuana a truly mysterious subterranean landscape.
When the cave was first discovered, it was reportedly filled with stalagmites, stalactites, and stone pillars, creating a spectacular underground landscape. Areas where sedimentary rock was exposed and sections where groundwater flowed also provided important opportunities for geological study.
Because of its scientific and natural significance, Oinuana Cave was designated a National Natural Monument on December 14, 1938.
Numerous animal bones, including those believed to belong to wolves, were also discovered approximately 15 meters from the entrance. Some of the bones were covered with deposits of cave minerals, suggesting that wolves may have used the cave over a period of several thousand years.
The remains also included the first brown bear bones discovered in Gunma Prefecture. These valuable specimens are currently preserved by the Ueno Village Board of Education.
After its discovery, Oinuana Cave was damaged by visitors who removed stalactites and other formations. Smoke and soot from torches and lanterns also stained the cave walls and formations. As a result, much of the cave’s original beauty was lost.
Many of the valuable animal-bone specimens are also believed to have been dispersed or lost over the years.
Today, Ueno Village leases the land from its owner and manages the site together with local residents. To protect the fragile underground environment, the entrance is secured with a metal gate and kept locked. Entry into the cave is prohibited.
Visitors can instead appreciate the entrance and surrounding natural landscape while learning about the cave’s history, legends, and scientific importance.
1908, Aza Nakagosawa, Otō, Ueno Village, Tano District, Gunma Prefecture, Japan
From Shinmachi Station on the JR Takasaki Line, take the Nihon Chuo Bus Okutani Line and get off at Koharu Bus Stop. From there, it is approximately a 50-minute walk.
Oinuana Cave is more than a limestone cave. It is a special place where natural formations, traditional mountain-dog beliefs, local folklore, and the story of its modern discovery come together.
Although visitors can no longer enter the cave, learning about its history and legends offers a way to appreciate the hidden value of this remarkable site.
When visiting Ueno Village, consider learning about Oinuana Cave alongside nearby Fudō Cave. The surrounding mountains and traditions provide a glimpse into an underground world shaped over thousands of years, as well as the stories that local communities have passed down through generations.