Investigation of the poles inside the grottoes also shows the possibilities of prior layout. The poles are generally triangular-shaped, and they are very regularly distributed in the caves. Once the position of the first cave is set, the positions of the other poles as well as the size of the cave are also set. Therefore, to guarantee the relative positions of different caves, the first cave's position had to be set beforehand.

But this is quite a complex problem and involves complicated underground orientation and vertical orientation. Did the ancient diggers successfully achieve this to the degree of perfection?

With the help of modern equipments and methods, the investigators measured the sizes of the walls, and surprisingly found that the overall construction is extremely precise. The underground orientation is also very accurate. The walls between the caves, sometimes thinner than one meter, are of the same thickness at different sections.

How did the ancient people dig 24 neighboring but unlinked caves in relatively small granite? And what kind of measure equipments did they use for the general layout?

Historical records show that the construction measurement devices usually included simple tools like a compass and plummet (a basic measuring tool). But were such tools enough to build the Longyou Grottoes? The experts are confused.

"There were definitely no modern measurement equipments like theodolite (a rotating surveying tool), gradienter (a tool to measure things such as the temperature and pressure when passing from one region to another) or GPS. Then how did the ancient people solve the problem of orientation? It remains a mystery," said Sun Jun.

Luo Zhewen, a seasoned expert in construction also thinks this as inexplicable. "The project must have been deliberately calculated, but how it was calculated remains unknown."

(Author: Jeff)

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