There is a 600 year old 240 paged book called the Voynich Manuscript, that’s written language is still completely unknown today.
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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
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.
16
There are pictures of Knights fighting Snails in old Medieval manuscripts - and nobody knows why.
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