The Platypus used to be considered a myth like Bigfoot and the Yeti.
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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.
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Scientists lit a cave of natural gas on fire in 1971 expecting it to only burn for a few days; it still burns to this day. It has been nicknamed "The Door to Hell".
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A waterfall known as The Devil's Kettle empties water into a hole and nobody knows where it ends up!
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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.
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