"The City of Is"

 Ys is a mythical city, said to have been situated on the coast of Brittany. Ys was created by Gradlon, King of Cornouaille, at the request of his beautiful daughter Dahut (or Dahud). Dahut tries to steal the key to the city from her father, (in some versions to give to a knight who turns out to be the devil). The city gate is opened and a giant wave then crashes down on the city, (either because God or the sea is angry at Dahut's sin, or because Dahut summoned it with sorcery), swallowing the city and sinking it beneath the sea. King Gradlon escapes on his magical horse Morvarc'h, but pushes Dahut into the sea where she becomes a mermaid. There are many different versions of the story, each with varying details and a different slant on the moral of the tale or who was to blame for the disaster.