8. Tomb of Genghis Khan, Possibly Mongolia
The tomb of famed Mongolian ruler Genghis Khan’s location is such a secret that it has yet to be discovered. After it was completed, it is reported that the slaves who built it were killed and then the soldiers who murdered them were also disposed of. It is rumoured that the 240 square kilometer area of the Burkhan Khaldun Mountains are the tomb’s possible site and there have been projects undertaken asking the public to tag potential locations based on images taken of the area from space. The discovery of Khan’s palace in 2004 led researchers to believe that they were close to discovering the tomb but they ultimately turned up empty handed.