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The glacial lake of Llyn Cau at Cader Idris is said to be bottomless and according to tradition it is thought to be the home of a lake monster responsible for drowning a man who went swimming in the cold waters. The story of the monster is probably related to an Arthurian tale concerning an Afanc from another lake, Llyn Barfog. An Afanc is a Welsh water dragon and this creature terrorised the locals living around Llyn Barfog. King Arthur captured the Afanc and tying it behind his horse, he dragged it to Llyn Cau, where he released it.