I'm not actually sure, though I believe it was a reservoir, which I think are man made?
We don't have very much information about it. I know there is a lot of rumour in the area (sunken trains, missing poeple...) but all we have on record before the last 20 or so years as a fishing area is a lifeboat test in 1866! It's all a bit of a mystery which is why I like going there so much