Mermaids, are they for real? Can you defend your answer?

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I have been fascinated by the supernatural since I was a kid. And the biggest puzzle of them all are the mermaids. I mean people do not believe what they cannot see etc etc right? But how do these extremely intelligent people, scientific know it alls, explain why around the world mermaids are known. FOR A FACT. Since the earliest time where travel is very limited how come that almost all people from different continents know of mermaids. Beautiful half woman half fish with long hair and beguiling voices? I mean if it was not really real how did this people who have never talked to each other have the same "myth" of a mermaid? All opinions are welcome. I rate all who answers seriously.