The Longyou Caves: What the Engineering Survey Found

Interior of a hand-carved Longyou Cave chamber in Zhejiang, China, showing stone pillars and uniform parallel chisel marks on the walls and ceiling

Most accounts of the Longyou Caves stop at "nobody knows who built them or why." That's true as far as it goes, but it skips the part engineers have actually figured out: exactly why ten of the twenty-four caverns are still standing after roughly 2,000 years while fourteen others collapsed, and why the surviving ones are now at real risk of following them. That answer comes from twelve years of fieldwork by a Chinese Academy of Sciences and University of Hong Kong research team, published in full in a peer-reviewed engineering journal. Readers of CurioLink's Göbekli Tepe coverage will recognize the shape of this story: the excavating specialists' own conclusions are more careful, and more interesting, than the "ancient mystery" framing usually allows.

The 1992 Discovery

In June 1992, farmer Wu Anai and several neighbors in Shiyan Beicun village, Zhejiang Province, pooled money for a water pump to drain a pond locals had always assumed was bottomless. After 17 days of continuous pumping, the drained pond turned out to be the flooded vertical entrance shaft of a massive hand-carved underground chamber. Four more nearby ponds were drained the same way, revealing four more complete caverns. Systematic surveying eventually identified 24 rock caverns total on a single small hillside, of which ten were found structurally complete and fourteen had partially or fully collapsed at some point before 1992.

Why Ten Survived and Fourteen Didn't

The engineering team's central finding is that the surviving caverns owe their condition to a specific design choice, not superior stonework. Every complete cavern was built with a vertical shaft entrance, which let it fill completely with rainwater and stay that way for two millennia. The fourteen failed caverns instead had horizontal, ground-level entrances, so water could simply drain back out — leaving their ceilings and walls exposed to open air and progressive weathering. Being fully submerged did two things for the surviving caverns: the water itself acted as a supportive medium that reduced stress on the surrounding rock, and — based on pH testing showing the local rainwater is weakly acidic while the argillaceous siltstone is weakly alkaline — a slow chemical reaction between the two appears to generate new calcite minerals that fill and self-heal hairline cracks before they can spread, a mechanism detailed in a dedicated follow-up study and consistent with earlier weathering research on the same siltstone. The researchers stress this is a documented hypothesis under continued investigation, not a fully closed case.

How Old Are They, Actually?

No historical document describes the caverns' construction, but three separate clues bracket their age. The earliest confirmed written reference is a poem by Yu Xun, dated between 1626 and 1676, which mentions the caves and therefore proves they existed by the early 17th century at the latest. Two additional poems, possibly referencing the same caverns, date to the Song Dynasty (960–1279 CE), which would push the confirmed reference back further still if the connection holds. Separately, in October 2001, workers clearing silt from Cavern No. 2 recovered two glazed clay pots identified as typical Western Han Dynasty ware (206 BCE–23 CE). The research team is explicit that this dating rests on an assumption — that the pots were deposited in the cavern around the time it was built, roughly 2,000 years ago — rather than on direct, independently confirmed dating of the excavation itself. No wood, soil, or organic material suitable for radiocarbon dating has yet been recovered from the cavern floors.

Who Built Them and Why — Still No Consensus

Chinese archaeologists have proposed stone quarrying, an imperial mausoleum, underground storage, a Taoist retreat, a secret military staging ground, a site for ancestor or deity worship, and a palace as possible original purposes. According to the engineering team's own review of the literature, "there is no consensus about the construction purpose of the large rock caverns," and each proposed use runs into objections from the physical evidence that no single theory has resolved.

Reconstructing the Construction Method

Without written records, the research team reconstructed likely construction methods from physical evidence alone. Small holes drilled into interior pillars and walls, spaced and angled consistently, are interpreted as sockets for firebrands used to light the underground workspace — and the complete absence of any soot staining on the finished carved surfaces suggests the decorative chisel-mark patterns were added only after each chamber's rough excavation was already complete, keeping the final surfaces clean. Four intact steel chisels recovered from Caverns No. 1 and 5 indicate the likely carving tool, paired with wooden mallets inferred from the tool marks. Two abandoned excavation pits found nearby, where diggers apparently hit unsuitable mudstone seams and gave up, suggest the builders used trial excavation to test rock quality before committing to a full cavern at any given spot.

What's Confirmed, and What Isn't

Documented by the 2011 engineering survey and related peer-reviewed studies: the 1992 discovery sequence; the ten complete versus fourteen failed caverns and their differing portal designs; the vertical-portal caverns' continuous water saturation as the primary factor in their long-term stability; the Western Han pottery recovered in 2001; the Yu Xun poem dated 1626–1676; the recovered steel chisels; and a confirmed, complete roof collapse of the already-failed Cavern No. 24 on September 30, 2010, which released roughly 250 cubic meters of rock.

Genuinely unresolved: the caverns' builders, exact construction date, and original purpose, none of which any current evidence source can establish with confidence; whether the two possible Song Dynasty poem references genuinely describe the same site; and whether the self-healing crack hypothesis fully explains the surviving caverns' integrity, which the research team itself describes as needing further validation.

FAQ

How were the Longyou Caves discovered?
In June 1992, villagers in Shiyan Beicun, Zhejiang Province, drained a pond long assumed to be bottomless and found it was the flooded entrance shaft of a large hand-carved underground chamber; four similar caverns nearby were found the same way.

How old are the Longyou Caves?
Their exact age is unconfirmed. The earliest solid written reference is a poem dated 1626–1676, and Western Han-era pottery (206 BCE–23 CE) found inside in 2001 suggests, but does not conclusively prove, a roughly 2,000-year age.

Are the Longyou Caves at risk of collapsing?
Yes. Engineering researchers who studied the site have warned that the five unwatered, tourist-accessible caverns are actively developing new cracks and could suffer a complete roof collapse without further preservation work — a fate that has already befallen one previously damaged cavern nearby, in 2010.