Atlantis was a mythical island that was said to have sunk below the ocean. It was first mentioned by Plato at around 350 BC, who wrote about a beautiful island in the Atlantic Ocean that was swallowed by the sea in one day and one night. He wrote two books about the history and culture of this mythical island.
Another sunken city was Ys, which legends claims reached back as far as the prehistoric era where a settlement was built off the coast of Douarnenez, France. The myth states that Gradlon, a King of Cornouaille, built a city within these walls on the request of his daughter Dahut who loved the sea. However, one day a wave as high as a mountain collapsed on Ys, dragging the city to the depths of the ocean.