Once upon a time there lived a beautiful mermaid in the clear blue waters of the Karersee. A powerful wizard named Masaré who lived in the mountains nearby saw her one day from his mountain lair and fell in love with her. Desperate to win her for himself, he sought the advice of the witch Lanwerda. She told him to disguise himself as a rich merchant and to cast a rainbow between the Catinaccio and Latemar mountains that overlooked the lake. This she assured the wizard  would lure the mermaid from the depths of the lake to the shore, from where he could woo her.
The wizard used his magic to cast a magnificent rainbow and conjure up treasure and fine clothes to dazzle the mermaid but in his haste forgot to disguise his face. As he approached the lake, the mermaid, who had come to the shore as the witch had foretold, saw the wizard and recognising him for who he was vanished with a flash of her tail. Furious, the wizard threw the rainbow and the jewels into the lake and left. The mermaid was never seen again but to this day the lake surface shines with the colours of the rainbow and it's depths with the colours of the jewels.