Longyou: Giant Underground Works Erased from Records, or an Ancient Project with Another Purpose

In June 1992, villagers at Shiyan Beicun in Longyou County, Zhejiang, China, drained five small ponds to catch fish. As the water fell, they found broad, dark spaces beneath the pond floors. Straight cut walls showed that people had made them.

Later surveys found nineteen more chambers near the first five, bringing the total to twenty-four artificial caves around Fenghuang Hill beside the Qujiang River. The builders cut them into argillaceous siltstone, a fine-grained sedimentary rock. It is softer than granite but can still hold large openings. The first studied caves were about 18 to 34 metres wide and 10 to 20 metres high. In some places, only about 0.6 metres of rock covered them.