It is said that the sister of Alexander The Great, Thessaloniki, turned into a mermaid after the death of her brother. The story goes that she wandered around the Aegean Sea in Greece and when she would see a boat, she would go close to it and ask its crew "is Alexander The Great still alive"? If the crew said "yes, he is alive and conqers the world" she would let them continue their trip. If they replied in a negative manner though, she would crush their boat and send them to their death.
Alchemy freediving creator Daan Verhoeven recently released a new video on his YouTube channel, where he captures a modern - much less feared mermaid - in Dahab. In his own words "This is Cath who is a freediver, yoga and scuba instructor in Dahab. Every once in a while, she swims behind a rock and transforms into a mermaid".