Rumored location: Douarnenez Bay, France
The legend: Another city swallowed by the sea for having too much fun. It was most beautiful in all of Europe, built by Gradlon, King of Cornouaille, off the coast of Brittany. He either deliberately built it below sea level (because his daughter loved the water so much) or took the throne after the sea had began to rise (nobody knows), but either way, only a massive gate could keep the water at bay. And the sole key to the gate was worn on a necklace around the king’s neck.
Now said daughter, Dahut, was known for being a pagan party animal. One night, she had a few too many and, seduced by a charming fellow who happened to be the devil, snatched the key from her sleeping father’s neck and opened the gate. Just about everyone died in the flood. The king somehow managed to get on his horse and swept up his beloved daughter to escape before realizing, hey wait, SHE was the problem -- and he pushed her in the water. Eventually, Gradlon gave up his pagan ways and converted to Christianity.