Gradlon the Great, king of Cornouaille of southern Brittany had the marvellous capital of Ys built for Dahut, his daughter. Because it was built below sea level, Ys was protected by great sea-walls , and only Gradlon had the key for the lock-gate.Dahut dreamed of a city where only riches, freedom and the joy of living would reign. So, she gave a dragon to the townspeople, which captured all the merchant ships at sea. Ys became the richest and most powerful city in Brittany. Dahut took a lover to her palace every night, and kill them in the morning with her magic, and bodies were then thrown from the top of a cliff into the ocean. One day, a prince dressed all in red came. Dahut immediately fell in love with him, but he was the messenger sent by God to punish the wicked town. He told her to steal the key to the lockgates from her father while he was asleep. So she did. As soon as she gave the key to the prince, he disappeared, then the water rushed in upon Ys. The great city had gone to the seabed instantly. It is said she was turned into a mermaid, to await the time when Ys would rise from the sea again. She sings with beautiul voice to attracts fishermen and kill them. They also said that when the weather is very quiet, people hear the bells ring from the disappeared city.
The city of Paris takes its name from Ys.

The Sunken Cathedral ( La Cathedrale Engloutie) , by Clude Debussy was written based on this story. What an inspired prelude.