Approximately 2500 years ago, a sector collapse occurred on Mt. Chōkai, causing a debris avalanche of around 6 billion tons of earth, rocks, and other material to flow down from the mountain. Trees were buried under the debris and stayed underground for a long time without any contact with the outside air.

These buried trees acted like a time capsule. Using a tree-ring dating method (dendrochronometry), researchers were able to estimate the date when Mt. Chōkai’s sector collapse occurred.

In 2015, 150 buried trees were found during construction of the Kisakata IC (an exit on the Nihonkai-Tōhoku Expressway). This area was the southern end of the debris avalanche in Kisakata. One of the trees found was a Japanese zelkova (Zelkova serrata) that was as much as 10 m long and 160 cm in circumference.

Buried trees and other items are on display at the Nikaho City Kisakata Local Museum. The buried tree that was found in 1994 has wedges driven into three places, which are from the 18th century (Edo period). A Japanese chestnut tree (Castanea crenata) was also found containing remains of ants from the Jōmon period (between 14,000 and 1,000 BC).