Llyn Tegid was formed during the last Ice Age when glaciers blocked the valley of the River Dee with debris. The resulting rectangular lake is 4 miles long, three-quarters of a mile wide and, in places, more than 42m deep. It's also the only home of the gwyniad, an endemic fish isolated in the glacial lake.
Local folk tales record an alternative to the geological version of events. Once upon a time the valley was home to a cruel and dissolute prince named Tegid Foel. One night the harpist at a banquet thrown by the prince kept hearing a small bird urging him to flee the palace. He did so, fell asleep on a hilltop, and awoke at dawn to find the palace and principality drowned beneath the lake.