For more than 2,000 years, the secrets of the mausoleum of Qin Shi Huang, China’s first emperor, were guarded by his Terracotta Army. But thanks to cutting-edge cosmic ray technology, archaeologists could soon discover what treasures were buried alongside him.
Cosmic rays can be used to scan a sealed tomb of the first emperor of China – it has long been rumored to contain deadly traps and an ancient map with rivers of liquid mercury.
Buried under a 249-feet-high pyramidal mound, the tomb lies within a necropolis in Xi’an’s Lintong District, and is famously guarded by the Terracotta Army. Found in their thousands to the tomb’s east, as if to protect Qin Shi Huang in death from the eastern states he conquered in life, each statue was once brightly painted.
However, exposure to the dry Xi’an air before appropriate conservation techniques had been devised meant that most of the soldiers’ colours faded after recovery.
For this reason, Chinese officials have long been reluctant to allow the tomb itself to be unearthed until they can guarantee the preservation of any artefacts within.
Qin Shi Huang conquered all kingdoms and ended the Warring States of China with one of the largest, most technologically advanced armies in the ancient world.
Emperor Qin launched the construction of the Great Wall, built a nationwide highway network and standardised writing, units and currency. But he was also criticised for brutal policies, such as burning unauthorised books and prosecuting Confucian scholars.
When the construction of the tomb was completed, all workers and craftsmen were trapped and killed to keep the secrets inside, according to Sima.
Yang Dikun, assistant professor of geophysics with the Southern University of Science and Technology in Shenzhen, who was not involved in the project but is familiar with the technology, said the project was feasible.
“The muon detector that we built and used for fieldwork nowadays has become so small, it can be carried around by a child,” he said.
Yang said archaeologists had tried to map the tomb using other methods. Gravity anomaly detectors could detect substances of different densities but its range was limited to a small area and its accuracy could easily be affected by environmental disturbances.
After decades of surveying, archaeologists have confirmed the existence of an underground palace more than 30 metres tall. They also found trace evidence supporting descriptions by Sima that had been disregarded as fairy tale, such as pools and waterways filled with mercury to mimic China’s major rivers and the sea.
But the palace’s detailed structure and the exact location of the emperor’s chamber remained uncertain. Sima’s other descriptions – such as traps armed with arrows and crossbows to shoot anyone who enters the tomb – were not verified.
Using cosmic rays in archaeology is a concept that dates as far back as the 1960s. Astrophysicists discovered that cosmic rays could hit air molecules and produce a particle known as a muon that could penetrate almost anything.
Muons have a higher chance of being absorbed when going through denser materials. By comparing the number of muons a detector received from various angles, archaeologists could discover hollow structures, such as hidden chambers or passages in a building.
But the idea remained largely theoretical because muons were not easy to detect. And for decades scientists had to rely on bulky devices as big as a room, making field application difficult.
In recent years, thanks to the advances in particle physics, the size of cosmic ray detectors has shrunk significantly. In 2017, an archaeological team in Egypt discovered a 30-metre-long chamber in a 4,500-year-old pyramid using a portable device.
“As an ancient civilisation with a long history, our country has a large number of cultural relics in urgent need of study. Muon imaging can be an important supplement to traditional geophysical methods,” Liu and her colleagues said in the research paper.
Electromagnetic signals were most sensitive to structures containing metals, and ground-penetrating radar had limited depth.
However, the cosmic ray approach is not without challenges. The detectors must be planted at appropriate depths without affecting the building or artefacts above, according to Yang.
And unlike other detection methods that could get results almost instantly, the muon detectors must be in place long enough to gather enough particle counts for analysis.
“You need to be patient,” Yang said.
Computer simulation conducted by Liu’s team suggested it could take a year to gather sufficient data to produce a clear image.
It is not clear whether – or when – the project will get the nod from the government to go ahead. Liu and her colleagues said some technical details, such as the exact number and location of the detectors, required further evaluation and optimisation.
Most imperial tombs in China have been robbed or damaged, but a few remain intact, including the tomb of Empress Wu Zetian of the Tang dynasty (618-907), who ruled when ancient Chinese civilisation reached its zenith.
Chinese archaeologists opened an imperial tomb of the Ming dynasty (1368-1644) in Beijing in the 1950s and witnessed precious materials such as silk and paper disintegrating soon after being exposed to the open air.
Since then China’s government has had a strict policy forbidding access to these sites.