SHIMANE, Jan 22, 2026 (News On Japan) - A rare prehistoric forest remains hidden beneath the ground in Oda, Shimane Prefecture, where the Sanbe Azukihara Buried Forest Park preserves giant trees that were buried roughly 4,000 years ago during Japan’s Jomon period.
While ancient cedar forests such as Yakushima are famous for their overwhelming presence, researchers say Shimane is home to an even more unusual natural treasure: a forest that was sealed underground while many trees were still standing upright, with their roots intact. The site is part of a museum and park complex known as the Sanbe Jomon Forest Museum and Sanbe Azukihara Buried Forest Park, which opened in 2015 and allows visitors to view the ancient forest deep below the surface.
At the facility, visitors descend to an underground exhibition room about 13 meters below ground, where dozens of massive trees can be seen in place. The site was designated a national natural monument in 2003, and experts say it is one of the most unusual examples in Japan of a “buried forest,” a term used for forests that remain underground in the position they originally grew.
According to specialists involved in excavation work, the forest is not fossilized or carbonized, but preserved as natural wood, with some samples still retaining a strong scent when cut. One cross-section sample contains more than 440 annual rings, and because it was taken from around 10 meters above the base, researchers estimate the tree may have lived for nearly 500 years, meaning it likely began growing about 4,500 years ago.
The reason the forest survived in such remarkable condition is linked to a volcanic eruption at nearby Mount Sanbe, which rises to 1,126 meters. Around 4,000 years ago, a major eruption triggered a large slope collapse on the mountain’s northern side, sending a debris flow down river valleys. The mud and rocks spread into the Azukihara valley, but by the time the flow reached the area, its force had weakened enough that many standing trees were not knocked down, even as their bases were buried.
Researchers say the site later became a natural dam area where water pooled and volcanic ash and sediment gradually accumulated, covering the forest deeper and deeper over time. This combination of weakened debris flow, rapid burial, and continued sediment buildup is believed to have created the rare conditions needed to preserve the trees in place for thousands of years.
The excavation process began after the first trees were discovered during construction work in 1983, when the exposed wood was initially treated as an obstacle. Several years later, a local high school teacher who saw photographs of the discovery realized that trees do not normally become buried upright unless an entire forest is preserved with roots still in the ground. His observation led to a full-scale investigation, eventually confirming the presence of a dense ancient forest.
In addition to towering trunks, the site has also produced evidence of the wider ecosystem from that era. Fallen leaves were found in layers, and insects have been discovered trapped between them, offering rare clues to the environment that existed when the forest was buried.
One of the most striking displays includes a massive root structure that appears to be a single trunk but is actually made up of multiple stems fused together. Experts believe it may have grown on top of an older fallen tree, gradually thickening and merging as the roots expanded to stabilize the growing trunk over centuries.
Researchers say the park offers visitors a chance to imagine the dark, dense forest that once stood there, and to reflect on how dramatically the landscape has changed since then. They hope the site will help people better understand both the power of nature and the long history hidden beneath the ground in Shimane.
Source: YOMIURI
