Kêr-Is (or Ys) was a mythical city swallowed up by the ocean.
King Gradlon ruled in Ys, a city built on land reclaimed from the sea, sometimes described as rich in commerce and the arts, with Gradlon's palace being made of marble, cedar and gold. In some versions, Gradlon built the city upon the request of his daughter Dahut, who loved the sea. To protect Ys from inundation, a dike was built with a gate that was opened for ships during low tide. The one key that opened the gate was held by the king.
Most versions of the legend present Gradlon as a pious man, and his daughter, Princess Dahut, as wayward. Dahut (sometimes called Ahez) is often presented as frivolous and an unrepentant sinner, or, sometimes, as a sorceress. However, in another version, that of an ancient ballad, Gradlon himself is blamed by his people for extravagances of every kind.
In most variations, Dahut acquires the key to the dikes from Gradlon, and its misuse leads to catastrophe. Commonly, Dahut steals the keys (made either of silver or gold) from her father while he sleeps, either to allow her lover inside for a banquet or after being persuaded to do so by her flattering lover. She opens the gates of the dikes, either in a wine-induced folly or by mistake, believing she is opening the city gates. ©