On the coast of Brittany, fishermen claimed to hear church bells ringing beneath the waves. They belonged to Ys, the low city built below sea level by King Gradlon, protected by a dike whose only key hung around the king’s neck, and drowned when his daughter Dahut opened the gates. This deep dive traces the legend from its earliest written mention in 1480 to a 2025 marine archaeology discovery near the Ile de Sein.

We show how Dahut is absent from Pierre Le Baud’s 1480 account, appears as a scapegoat villain in a 17th-century edition of Albert Le Grand’s lives of Breton saints, and becomes a dragon-taming sorceress seduced by a red-bearded devil in Emile Souvestre’s 1844 version. We then follow the myth into Debussy, Lalo, and video games, and explain how submerged Neolithic structures reveal a real prehistoric climate disaster beneath the fairy tale.

• The single silver key: how the dike’s security became a legend of catastrophic human error
• Gradlon’s flight on horseback and the saint’s order to throw the demon into the sea
• Corrigans, silver grain goblets, sea dragons, and a devil playing bagpipes
• Dahut as mermaid, the roaring giant beneath the bay, and the saying about Paris being engulfed
• Sonar, sediment cores, and ruins up to 10,000 years old: what the discovery means for modern coastal cities