The other obvious problem is lighting. Because of such great depths, some corners at the bottom of the Longyou Grottoes are pitch-dark, thus making the delicate carving of the patterns in the caves almost impossible to complete. Then how did the ancient people solve this problem?

According to Jia Gang, a Tongji University professor specializing in civil engineering: "There should be lamps, because the cave's mouth is very small, and the sunbeam could only shine in the cave at a certain angle during a certain period of time. As one goes deeper to the cave, the light becomes dimmer. At the cave's bottom, which is usually dozen of meters from the mouth, one could hardly see anything."

The builders of other ancient caves used lamps. For instance, a lot of obvious traces of burned oil have been found on the walls of the Flower Mountain (Huashan) Grottoes in Huangshan City of East China's Anhui Province . Lamp bases, oil plates and other lamp equipment have also been found near these traces.

But the bewildering thing about Longyou Grottoes is that there are absolutely no traces of fire nor any lighting equipment in it. Is this just a result of centuries of water erosion? But generally speaking, the carbon black, as a result of fire, is highly stable. Once it sticks to the stone, it is very hard to be washed away by water.

Does this indicate that the ancient people built the Longyou Grottoes only with the help of natural lighting?

A closer look at the grottoes' structure will show that all the mouths are located at the grottoes' southwest, which ensures the sunbeam would shine inside all day long. Also, during the noontime, one can see almost everywhere. So did the ancients simply use this natural lighting? No widely-recognized agreement has been made so far about the lighting problem.

(Author: Jeff)

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