Why Didn't the 1,600-Year-Old Timber Decay?

Jangmisanseong’s Wooden Water Facility Preserved by Perfect Seal

Meticulous Proportions in Construction... Yet Buried After Only a Century

Institute Expands Excavation to Summit Residential Area

Excavation Report Meeting at Jangmisanseong, Chungju / Cultural Heritage Administration.

"Can you believe that this timber is 1,600 years old?" On September 1, at the public excavation site of Jangmisanseong Fortress in Chungju, Choi Kwanho, Special Researcher at the National Jungwon Research Institute of Cultural Heritage, pointed to the wall of the wooden reservoir and posed this question. The chestnut and oak wood, with its grain still clearly visible, appeared exceptionally well-preserved—so much so that it could have been mistaken for freshly cut timber from just a few years ago.

The reservoir was first revealed in November 2024. Since beginning field investigations in July 2022, the institute identified timber submerged in water in the valley and began excavation. The excavation did not commence immediately; instead, during the challenging winter period for excavations, they thoroughly reviewed domestic and international cases of ancient wooden reservoirs. This approach was taken out of a strict principle to avoid damaging the facility.

There are clear reasons why the reservoir’s timber endured for so long. As the interior became filled with mud and groundwater, fine layers of sediment accumulated above. Subsequently, incoming soil completely sealed off the structure. Choi explained, "The main factors that damage timber are direct sunlight and exposure to oxygen," adding, "The long period the wood remained buried in soil blocked these risk factors.” However, a new risk emerged after the excavation: as soon as the timber surfaced, those threats returned. Therefore, even before starting the investigation, the institute constructed protective facilities to block direct sunlight.

Wood Used in the Wooden Reservoir House at Jangmisanseong Fortress in Chungju / Cultural Heritage Administration.

The timber forming the reservoir walls averages 7.2 meters in length and about 20 centimeters in width. The stacking method differs from conventional techniques—whereas ancient underground wooden structures typically use the “inter-column” technique of erecting posts and inserting planks between them, the Jangmisanseong reservoir stacked long timber horizontally layer by layer.

The foundational timbers at the base were joined in a well (“井”) shape. Choi elaborated, “In the well-shaped base, the two horizontal beams create three spaces with a consistent ratio of 1:2:1 on both sides.” He added, “This indicates that the Baekje people meticulously calculated which length and thickness of wood would be used for each wall and location in advance.”

The reservoir is located where the valley water, flowing north from the summit of Jangmisan, collects before passing over the fortress walls. Even now, groundwater continues to spring up, and unless drained, the water level nearly refills by the next morning. Right beside it stands a smaller, stone-built water collection facility. According to the institute, the structure was designed so that water from the large reservoir is filtered once before seeping into the smaller one, likely to secure a separate source of clearer water.

Wood Used in Wooden Reservoir at Jangmisanseong, Chungju / Cultural Heritage Administration.

The institute is filtering the soil accumulated inside the reservoir with water to recover micro artifacts and organic materials. Out of a total volume of 83 tons, about a quarter has been processed so far. As the excavation enters its final stage, the plan is to speed up the water-sieving process over the next three to four months.

Some questions remain unanswered—chief among them, why such an elaborately constructed facility ended up buried in less than a century. The institute tentatively assumes that after Baekje’s withdrawal, the group that took over this area repurposed the fortress, rendering the reservoir unnecessary. During this process, the walls of the fortress are believed to have been converted from earthen to stonework. However, the institute has not identified whether this subsequent power was Goguryeo or Silla.

The institute believes that the main living spaces were located at the summit of Jangmisan rather than around the reservoir, given that the peak features a flat area about 30–50 meters wide and 200 meters long—a likely site for administrative or military buildings and small to mid-sized residential areas.

Jangmisanseong Wooden Reservoir Root View in Chungju / Cultural Heritage Administration.

A gentle slope connecting the peak to the reservoir valley is presumed to have housed storage and iron production facilities. This conclusion is based on the discovery of slag (residue from smelting ore) and fragments of air-blowing pipes during the recent investigation. Choi stated, "Once the ongoing investigation is complete, we plan to gradually study the full extent and internal facilities of the Baekje earthen fortress, focusing on the northern sector of Jangmisan."

This content was produced with the assistance of AI translation services.