Mermaids (or Sirens) are mythological water creatures (or spirits) that have appeared in the folklore and popular culture of almost every sea fearing civilizations during the last few thousand years. Often depicted as beautiful women with lower portion of body resembling fish tail, these legendary creatures managed to create around them both the atmosphere of romantic myths and horror stories. In the beginning, mermaids represented the unknown of the sea, dangers of open water, and unexplored water territories, but as our culture and science evolved to modern state, mermaids and sirens took a firm hold in our imagination and became part of art and media.
As the early human civilization formed around the rivers and seas, their religion often had a great focus on the dangers and wonders that were hid in them. Because of that some of the earliest known gods were depicted as some combination of men and fish, with 7000 year old Babylonian god Ea (bringer of knowledge, arts and sciences, later known as Oannes by the Greeks) being first one. As the time went on female sea gods appeared, and the first one that had the greatest resemblance to the mermaid was Assyriangoddess Astargatis, who decided to hide herself from the mortals after she accidentally killed one of them. After diving below water to become fish, sea refused to hide her beauty and decided to not transform upper half of her body. Greeks adopted some parts of her origin, morphing her into Aphrodite. The clearest example of merfolk in Greek mythology was the son of Poseidon and Amphitrite called Triton, who is most often shown as mermen who blow the conch shell while riding the sea waves.
As the myth about mermaids spread across the world, various cultures adopted many traits to them, sometimes creating creatures with vastly different attitudes toward the mortals. Greeks for example feared mermaids, and they called them sirens - dangerous creatures who lured the sailors to the sea with their songs and drowned them. Similar superstition was present on British Isles where sailors regarded mermaids, sirens and sea nymphs as evil spirits of the sea and bad omens (sighting the mermaid represented the coming of the storm, sinking of the ship and almost always eventual death of the person who saw them).
Many however accepted mermaids as the good willed creatures that are extremely shy but also very curious about life above the sea. Chinese legends speak that tears of elusive sirens form the most beautiful pearls on Earth, Chinese sailors thought that sirens can grant immortality to the worthy man, and Irish held belief that mermaids are calling the sailors to the sea with their songs as the sign of love. Persian viewed mermaids not as half fish, but also as complete human beings that were able to live in the sea. In the many stories that were written about them (even in the famous collection of folk tales One Thousand and One Nights) there are depicted as beautiful men and women who can have children with ordinary people, and their children can also live in the sea if they want so.
Another source of mermaid popularity was their elusive nature and constant stream of sightings that was promoted in media. Dozens of highly publicized sightings from all four corners in the world kept the myth of the sirens fresh in the public minds, and even fake or manipulated findings of dead merfolk bodies did not managed to shatter their popularity.
The biggest driving force of mermaid popularization came in 1836 with the fairytale "The Little Mermaid" written by Hans Christian Andersen. This work of fiction that describes the quest of mermaid princess to come to the land solidified mermaids as the popular mythological creatures, and all the future media work had this fairytale as their basis.