This is Dozmary Pool high up on Bodmin Moor near to Jamaica Inn at Bolventor. This is where Sir Bevedere returned Excalibur to the Lady of the Lake after King Arthur's last battle. Another legend to do with the lake (which is supposed to be bottomless) is a Jan Tregeale was set a punishment for having a pact with the Devil. His task was to empty the lake with a leaky limpet shell. Jan gave up and run away but was set another equally impossible task at Roche Rock. I think the lake has been known to have dried up in the past.