Marie Trevelyan*, in 1909, reported the following folklore about a Pwllhelig in the Vale of Glamorgan, which is probably that in Cardiff...

Pwllhelig Pool, in the Vale of Glamorgan, was frequented by a lady robed in black, with a long veil and trailing garments. She was heard murmuring to herself, and sometimes moaning. In the eighteenth century people said that a house once stood on the spot now occupied by the pool. It was under a ban, and for the evil deeds of the owner the house went down in a gap or landslip, which afterwards, owing to the bursting of a spring, was filled with water. The lady was supposed to be the wife of the evil owner, and she was always hoping to find her lost money and jewels. Early in the nineteenth century a man undertook to speak to her. She did not answer, but only smiled sadly and wrung her hands, then vanished.

If this is the Dark Age pool, it just goes to show how reliable oral culture can be!