This cave, now called the Longyou Caves, represent one of the largest underground excavation made during ancient times. A total of 24 hand-dug caves were eventually discovered, each with an average floor area of a thousand square meters and ceilings that reaches heights of up to 30 meters. The total area covered by all the caves exceeds 30,000 square meters.
We know the caves are not natural because the interior walls are covered with chisel marks. The chiseling was also done in a such a way so as to leave a uniform patterns of parallel grooves which some people believe is symbolic. The markings are similar to those found on pottery housed in a nearby museum, which is dated between 500 and 800 BC. There are also staircases, pillars and exquisitely carved figures.
According to some estimate the caves were dug more than 2000 years ago, possibly in 200 BCE. A rough estimate reveals that nearly a million cubic meters of rock were removed to create the caves. A project so large would surely have been mentioned by historians and travelers, but no documents whatsoever have been found, which is especially odd considering the ancient Chinese were meticulous record-keepers.
However, one cannot rule out the possibility that the ancient cave builders were simply enlarging an existing natural cave which would explain a lot of things, such as how they were constructed and what happened to the excavated rocks. Nevertheless, the excavation of the caves were a monumental project executed by man and will continue to be an enduring mystery.
Grooves on the walls and ceilings.
Source : China Culture