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
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.
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There is a 600 year old 240 paged book called the Voynich Manuscript, that’s written language is still completely unknown today.
15
The Platypus used to be considered a myth like Bigfoot and the Yeti.
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In 1987, a Chicago TV station had its broadcast interrupted by a pirate signal that shows a man in a Max Headroom mask mumbling and being spanked with a fly swatter. Despite investigations by the FCC and FBI, this became the only known incident in which a hacker interrupted a major TV broadcast without getting caught.
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