The Isles of Scilly were formerly one island and many archaeological sites now lie beneath the sea. Excavations have only been possible at the lowest tides and there are certain to be more sites permanently under water (there are more than 500 sites above the high water line). Normally submerged sites that have been excavated include: 10 hut circles, 7 cists and graves, 4 field wall enclosures and 12 other occupational sites and partial exposures in eroding cliffs. See marine contour map.
There is a persistent legend that the Lost Land of Lyonesse once stretched from Scilly to Land's End. This realm was said to be the home of Tristan who went on to woo Iseult etc. The capital of Lyonesse was the City of Lions and was built around the hill which is now the treacherous reef of the Seven Stones. One night a huge wave (tsunami?) swept over Lyonesse and only one man escaped. This was Trevilian who galloped ahead of the flood on a white horse and survived to found the Cornish Trevelyan family.
There are many field walls and hut circles to be seen at the lowest tide when it is possible to walk from Samson to Bryher and from Bryher to Tresco. In Roman times, all of the islands were one (variously referred to as Sylina Insula and Siluram Insulam - singular, not plural ie The Scilly Island) with the exception of St Agnes and Annet. This has given rise to the legend that Scilly is a remnant of the Lost Land of Lyonesse. The legend further relates that the rest of Lyonesse lies beneath the sea between Scilly and Lands End and over towards the Lizard. There really is a submerged forest in Mount's Bay and fishermen have reported seeing the tops of houses near the Longships lighthouse. The legend is further related in Cheryl Straffon's excellent guide to Ancient Sites on the Isles of Scilly (Meyn Mamvro). Whatever else may lie beneath the waves, there are the remains of hundreds of shipwrecks awaiting the marine archaeolgist.