Chinese archaeologists have found a group of huge rings at the site of the 1,300-year-old tomb of Wu Zetian, China's only empress, but they are unable to explain their existence.
At least 10 rings appeared on aerial photographs taken by experts from the Xi'an Preservation and Restoration Center of Cultural Relics and Qianling Museum in a survey of Qianling.
Most of the rings were 30 to 40 meters in diameter and were in a zone four kilometers long from east to west and two kilometers from south to north, said Qin Jianming, a researcher with the center.
The most eye-catching was the largest ring, with the diameter of 110 meters sited in fields, he said.
Qin said the foundation of the largest ring was three meters thick, and the color was distinctly darker than the fields. Three quarters of the ring were clearly visible, despite being broken by a footpath.
"At first we believed the rings were atmospheric phenomena caused by lights, but after analysis and comparison with previous aerial photos, we are sure they are historic remains," he said.
Located 80 kilometers northwest of the ancient city of Xi'an, capital of northwest China's Shaanxi Province, Qianling is the joint tomb of Wu Zetian, who ruled for 50 years, and her husband, Emperor Li Zhi of the Tang Dynasty (618-907).
It is the only tomb in China which contains the bodies of two rulers. Wu Zetian was buried in the tomb 22 years after her husband.
Qianling is the best preserved ancient tomb in China and has not been looted.
The researchers conducted a ground search for the rings three to five kilometers from Lianshan mountain range, which was the territory of Qianling, and they were difficult to see from the ground.
"The layout of the rings seems irregular and some are no longer intact. We are assuming there were more rings in the past, but they have eroded due to geographical conditions and human activities," said Qin.
Despite searches of historical records, the researchers still have no idea of the purpose of the rings or their relation with the tombs.
"Based on our experience, we assume they were tunnels built in ancient times," said Qin.
However, the center of the largest ring was an excavated tomb belonging to a ruler of the Yan State in the Tang Dynasty, which was a subsidiary tomb of Qianling, but it was uncertain if the ring was part of the tomb, said Fan Yingfeng, head of the Qianling Museum.
"Human activities in that area began in New Stone Age. Emperor Qin Shihuang built a palace there and rebellious forces in late Tang Dynasty occupied the area as well," he said.
Experts said the discovery indicated there might be more historic relics yet to be discovered in the Qianling area.