The canon is seen partly as Emperor Yongle's way of declaring the legitimacy of his reign because he was controversial as a usurper. [Photo provided to China Daily]

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.