How do the fruits of what is thought to be one of the largest publishing exercises ever undertaken simply vanish? Chances of finding the answer to that question seem to be diminishing with time
It is believed to be the largest paper-based general encyclopedia ever written, presented on 22,937 rolls in 11,095 volumes and using 370 million Chinese characters.
That means if someone had managed to finish reading one volume of The Great Canon of the Yongle Era, or Yongle Dadian, every day it would have taken them a full 30 years to polish off the lot.
Only two copies are known to have existed, the original one commissioned by Zhu Di, known as Emperor Yongle (1360-1424) of the Ming Dynasty (1368-1644) in 1403, and a transcript completed during Emperor Jiajing's reign (1521-66), all handwritten.
Each of the two copies is believed to have taken up about 40 cubic meters - the capacity of about 320 medium-sized cartons, or a small room - yet somehow the original one disappeared.
Only about 400 volumes of the transcript version, less than 4 percent of the total, were collected worldwide. So have the others survived, and if so where could they possibly be?
An exhibition at the National Museum of Classic Books in Beijing since September brings to light anecdotes behind how the masterpiece was put together, preserved, copied and then disappeared, and conjecture about the whereabouts of the original.
Twelve transcript volumes, together with rare historical documents, rubbings, manuscripts, archives and photocopies of the canon published in different periods and collected overseas are on display.
The volumes are from the collection of the National Library of China, which is now home to 224 volumes - more than half of the existing ones. Sixty-two of the 224 volumes are temporarily kept at the Taipei Palace Museum.
The canon was a compilation of ancient Chinese documents that included books - possibly as many as 8,000 of them - written between the pre-Qin days (before 221 BC) to the early Ming Dynasty, covering literary classics, astronomy, geography, medicine, divination, theater, crafts and agriculture, among others.
The canon was especially abundant with materials regarding the Song (960-1279) and Yuan (1271-1368) dynasties.
Each volume was 50.3 cm tall and 30 cm wide, mostly including two rolls. Stencil tissue paper and ink stick produced in Huizhou, now in Anhui province, were applied, considered the best choice for handwriting.
Thus, much of the blank space in the existing volumes was cut out for private use or imitating scripture paper later in history, causing severe damage to the canon.
Instead of simply being a compendium of ancient books, the canon was arranged more like a dictionary. The ancient documents were distributed by paragraph or by passage, rearranged and attached to different entries.
Each entry started with a certain Chinese character, followed by any materials related to the character, such as phonation, earliest record, meanings and illustrations of what they looked like in different Chinese calligraphy fonts.
All the previous records that included the character would be applied, including their references and authors, so many later scholars relied on the canon to collect lost documents, which became one of the canon's most significant aspects.
It has also provided an argument that the Venetian merchant and traveler Marco Polo (1254-1324) did indeed visit China, something that is a matter of dispute, says Yang Yinmin, a research librarian at the National Library of China.
According to The Travels of Marco Polo, after living in China for 17 years, Polo proposed returning home but was rejected by Kublai Khan, emperor of the Yuan Dynasty at the time.
However, together with his family, Polo escorted three envoys to Persia and eventually returned home.
This account and the names of the three envoys were matched against the record in the 19,418th roll of the canon, persuasive evidence that Polo had been to China, Yang says.
It was difficult to keep books in ancient times and some were barely known to the public. Collecting such a huge number could be an incredibly difficult and expensive undertaking.
So compilation of the canon reflects the dynasty's confidence and power, says the best-selling historical novel Ups and Downs of the Ming Dynasty by Dangnian Mingyue, the pseudonym of Shi Yue.
The canon is seen partly as Emperor Yongle's way of declaring the legitimacy of his reign because he was controversial as a usurper.
Historical records show that Emperor Yongle asked the compilers "not to be bothered by the length".
The officer in charge of the compilation, Xie Jin (1369-1415), was known as "the most talented man in the Ming Dynasty" for his broad knowledge, strong discernment, unparalleled patience and inclusive thought, Ups and Downs of the Ming Dynasty says.
In 1403 Xie organized 147 scholars to do the compilation and they turned in a draft one year later. However, Emperor Yongle thought more was needed.
Then in 1405 he designated his henchman Yao Guangxiao to lead the work and mobilized more than 2,000 intellectuals, many of whose handwriting was exquisite, to enlarge the canon.
As they occupied themselves with the compilation, all their worldly needs were looked after.
After the masterpiece was completed in 1408, the emperor was so delighted with what he read that he wrote a personal preface to it and named the collection The Great Canon of the Yongle Era.
However, few of his successors took much interest in the canon until Emperor Jiajing, who frequently had several volumes at hand and often cited the canon in court.
The night the Forbidden City caught fire in 1557 he was so concerned about the canon's fate that three times during the night he issued decrees calling for all to be done to save it.
In 1562 Emperor Jiajing finally made up his mind to have the canon transcribed. Those working on it included Zhang Juzheng, a reformer and one of the most famous prime ministers of the dynasty.
In addition to a group of top scholars, another 109 people joined in on the transcription work. In fonts and type size the transcripts were consistent with the original. To assure accuracy, each of the transcribers was allowed to write down only three pages a day. Their bylines were left at the end to ensure the lines of responsibility were clear.
It took them five years to finish the work.
For all the conjecture that surrounds the fate of the missing canon, as time goes on the chances that it will ever be found seem to diminish. However, Emperor Jiajing's devotion to the transcript gives clues to one of the major theories.
The emperor was buried three months after he died. In a month, his successor announced the transcript was completed, and it seems that from then the original disappeared without trace.
Four years before Emperor Jiajing died he urged a courtier to speed up the transcript and is said to have emphasized that the two versions would be kept in separate places.
So Luan Guiming, a researcher at the Chinese Academy of Social Sciences in Beijing and Zhang Chenshi, former editor of Zhonghua Book Company, have speculated that the canon was buried with the emperor in the Yongling tomb in northwestern Beijing.
Yao Zhenxing, academician of the Chinese Academy of Sciences, and Wang Qianshen, a researcher at the academy's Institute of Geology and Geophysics, proposed in the 1990s that a high-precision gravimeter be brought in to search for a possible underground palace on the site of the Yongling tomb.
A high-precision gravimeter survey would be unable to discern what was inside the tomb, but it would be able to detail the distribution and scale of an underground palace.
They reckoned this could lend weight - or otherwise - to the theory that the underground palace included extra rooms - just as its counterpart found in the nearby Dingling tomb, which belonged to Emperor Wanli, Emperor Jiajing's grandson, did - that might hold the lost canon.
However, the proposal was denied because, it was said, archaeological and preservation technology of the time could not guarantee appropriate conservation of imperial mausoleums, and exploratory work was stopped.
In addition, some experts say the Yongling tomb is waterlogged, so even if the canon were there it may not have survived.
Other speculation has it that the canon was burned in a fire in the imperial palace at some time or other.
Although few volumes of the canon remain, it continues to inspire many of those looking into Chinese classical documents.
According to a China Central Television documentary, the way the canon was arranged has formed the framework of a digitization project for Chinese classical documents Luan leads that started more than 30 years ago.
Whether the canon is intact or not, the way the canon is arranged is important, Luan says in the documentary.