First of all, every emperor began to build his own mausoleum after he ascended the throne, so the longer the emperor was in office, the bigger his mausoleum would be. The tombs of emperors who were in office for a short time were also built by their superiors and then registered. For example, it took 53 years to build the mausoleum of Emperor Wu of the Han Dynasty. So why didn't there be a mausoleum in the Yuan Dynasty? This is because the funeral custom in Yuan Dynasty is different from other dynasties. Although there were many ethnic minorities in the Qing Dynasty, with the passage of time, the Qing Dynasty has gradually been sinicized, so their funeral system is modeled after that of the Ming Dynasty. The Yuan Dynasty was founded by nomadic people, and their funerals generally followed the words "light burial, simple mourning". And most importantly, they are nomadic people. We all know that nomadic people have no fixed residence, so they can't find a fixed place like Han people, watch it expand for many years, and then bury it there after death.
Secondly, after Yuan Di buried them, there will be a special army to protect the place where they were buried from outsiders, and then a large number of horses will repeatedly beat the arrow towers on the buried tombs until the mounds above the ground are trampled flat, and then the army protecting the tombs will always guard the tombs until the dense grass growing on the tombs is not much different from the surrounding terrain, and no special traces can be seen.
Finally, the above content only represents Bian Xiao's personal views. Please forgive me if there is any mistake.