Lyonesse was once a land between the Scilly isles and Cornwall. It is where Guenevere came from. Sailors have reported seeing things under the sea and even dredging Artifacts up in thier nets. Some say they hear church bells ringing under the sea, some say they can be heard from the coast of Cornwall. Unless someone actually does a survey of the area like they did around the Dogger Bank, we will never know.

Some say that Glastonbury Tor is the Isle of Avalon. That many years ago the land around it was a flood plane under several feet of water.

I was here at Glastonbury Tor in August 2012. I was not able to climb up there due to time restrictions, some day I have to go though. Its on my bucket list. I better not wait too long or it may well be the last thing on my list. Phew!!! The Tor is in the midst of "summerland Meadows" now if that is not Arthurian I don't know what is.
the Tor once rose like an island from the waters, but now, with the surrounding flats, is a peninsula washed on three sides by the River Brue. The remains of Glastonbury Lake Village nearby were identified in 1892.

Dozemary pool is one site that is claimed to be the home of the Lady of the Lake. According to the legend, it is here that King Arthur rowed out to the Lady of the Lake and received the sword Excalibur. The pool is also the place where Bedivere returned Excalibur as Arthur lay dying after the Battle of Camlann.
There is also Tintagel............with Merlins Cave close by (in another post) ...Legends abound. When you go to these places, walk them and look in silence, its not hard to imagin and for me, well I believe there is truth in legend. Some tales have been handed down almost intact and have been cherished and written about since just after the events. Who can say they are not true?