Some are more desirable because every person has a personal preference that appeals to them and if it isn't appealing to them or if society likes one more, it would naturally draw the to it.
I think it’s both ice cores and tree rings
The Mongol Empire, with its peak size under the rule of Kublai Khan around the year of 1279, included parts of modern Russia in the North, modern day Korea, and modern China in the South on one end of the Asian continent. It stretched through the Asian continent all the way to modern day Hungary in Europe. It also included parts of the modern Middle East such as Iraq and Iran. The total area of the Mongol Empire in its peak was around 40 million km^2.
