The exact cause of death and burial site of Genghis Khan is unknown, because all of the 2000 funeral guests and 800 soldiers were killed in order to preserve the secret.
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The Platypus used to be considered a myth like Bigfoot and the Yeti.
18
Since 1982, a radio frequency emanating from Russia has been broadcasting a buzzer-like sound and, periodically, an unknown voice reads orders, names, and numbers. No one knows where exactly it is coming from or what it means.
13
A waterfall known as The Devil's Kettle empties water into a hole and nobody knows where it ends up!
18
There is a 600 year old 240 paged book called the Voynich Manuscript, that’s written language is still completely unknown today.
15
There's a mysterious dwarf planet between Mars and Jupiter called 'Ceres' which has never been visited by spacecraft or photographed in detail, however, Earth-bound telescopes reveal a large bright shining spot on the surface of this planet, the origin and nature of which are unknown.
5