U can choose telling about Atlantis or Bermuda triangle.
Mystery is something story which is remaind unexplained u can say about the bhangar fort in rajasthan ehich is a mestorios place few years back 15 ppl were dead in that fort and 5 or 6 months back 10 more ppl were dead and intresting fact is tat no house can be constructed near tat for if u it will collaps story of this fort is known but mystery behind this fort is unknown add ur own points and pesnolize the topic explain any mystery in ur life if  u have all the best do well !!!!!!!!!! :)
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