In the Celtic mythology of Brittany Ker Ys (Breton: Kêr-Is) was a City lost under the sea. The City was said to have been built by Gradlon, legendary 5th century King associated with the area of Cornouaille in Brittany. King Gradlon was said to be a great warrior and was engaged in warfare against lands in the far north. He had many ships which he used to conduct his attacks. Although successful his sailors became tired of fighting and decided to return home leaving Gradlon alone in the north. It was then that Gradlon met the magician and Queen of the North, Malgven.
Impressed by his prowess as a warrior, she persuaded Gradlon to join her in killing her husband, the King of the North. The pair then took Morvarc'h, Malgven’s magical black horse, who is described as being able to ride on the sea and breathed fire from his nostrils, One version say’s that they caught up with the fleet of boats carrying Gradlon’s sailors, but that a storm separated them from the main fleet. Another says that the fleet scattered out of fear of the site of Morvarc'h. The result of either was that Gradlon and Malgven were adrift for many months. On the long journey Malgven gave birth to their daughter, Dahut. In some versions of the story Malgven died during childbirth. In others, she did not die but decided to return to her own land having established that Gradlon loved and cared for the child. She asked Gradlon to drop her off on an island and told him that Dahut would always have Malgven’s face to remind him of her.
On his eventual arrival in Brittany, Gradlon, with the encouragement of Dahut, built the marine city of Ys in the bay of Douarnenez, which was below sea level at high tide. Another version is that the city had already been in existence two centuries before the arrival of Gradlon. The city was protected from the sea by a dam with a gate to which only the King had a key. However, the Princess Dahut sought to turn Ys, which was increasingly coming under her influence, into a city of wealth, unbridled pleasure and excitement. It is said she arranged to have all merchant vessels sailing in the vicinity seized and the cities riches grew. She was beautiful and took a new lover every night. However, each lover was made to wear a silk mask which was enchanted and in the morning the mask killed them.
It is said that the behaviour within Ys met with the disapproval of St Winwaloe (c460-532A D) who was born in Brittany and son of a Cornish prince. St Winwaloe founded Landévennec Abbey. He denounced the debauchery taking place in Ys, but this was ignored by Dahut. One day a knight dressed in red armour arrived in Ys and Dahut became besotted with him. He persuaded her to steal the key to the dam gate from King Gradlon and give it to him. On a stormy night when the tide was high he opened the gate and the city became flooded and submerged into the sea. It is said that the red night was the devil. King Gradlon and his daughter Dahut escaped the city on the magical horse Morvarc'h. As they rode over the tempestuous sea St Winwaloe came to them and demanded that King Gradlon abandon his daughter. The reluctant King did so, saving himself but condemning Dahut to the sea. The legend is she became a morgen, the siren like mermaids with the power to lure men by their beauty and drown them.
This was how the great Breton city of Ys was lost under the sea. Although on still, calm days off the coast of Brittany, locals will tell you that they can hear the muffled sounds of the city’s bells tolling far beneath the waves.