Mermaids are part of folklore for many centuries. First known writings about them are actually about four thousand years old! These amazing half human half fish creatures live under water, not only in oceans, but in lakes and rivers as well. Without water there is no life, but water with its mysterious depths and raw power always presented some kind of danger too.
Humankind constructed many tales and legends about weird beings praying beneath the water surface and mermaids are among most popular ones. It's interesting to note they are humans from waist up everywhere, but have a fishtail only in southern parts of the world with Mediterranean on the top of the list. It seems Scandinavian folklore which of course has its own mermaids and mermen incorporated this element only in Middle Ages. Before that these mythological creatures were more humanlike, they just lived under water.
There are also other details by which mermaids from one part of the world differ from mermaids from other parts. Some liked to meet people, especially young men, and they convinced them (sometimes by their physical appearance, sometimes by enchanting songs and sometimes by offering them treasures, like lumps of gold) to follow them under water, where they drown.
Some mermaids did that because they didn't realize people can't breath in their underwater kingdoms, other because they collected their souls, and others just because.
History is full of reports of seeings of mermaids, some as extremely beautiful girls and some as huge monsters. Many fraudsters made good money on displaying remains of mermaids which were made
of monkey skeleton sewn with a fish tail.
Today, after all these years, mermaids are still popular. One mermaid supposedly live near Copenhagen and one near Warshaw. These cities have them for the protectors.
They also inspired numerous great stories and other artistic works.