Sea level around Britain is constantly changing, not just with the daily fluctuations in tide. The current trend is for a steadily rising sea level. This swamped Doggerland in the North Sea and has given the southern edge of Britain a submergent coastline.
The Scilly Isles have been affected in the same way.
Apart from St Agnes, which is separated from St Mary’s by a deep-water channel, the main islands of the Scilly group only have shallow water between them. Indeed, at a very low spring tide it is possible to walk across the sand between Bryher and Tresco.
Take the sea level down by just 8 metres and you get a single island (Ennor) which was twice the current land area.
As the sea level rose, the low land between the islands was gradually swamped. The population had to move, abandoning settlements and retreating to the higher ground.
Many historical sites were submerged, to be later rediscovered and explored by divers.
As recently as the 11th century, the islands were still joined at low water. This is reflected in local place names, with the early Cornish language used on the outer edge of the islands, and places on the inward shores, bordering the shallow sea, having names with English roots, given since 1600.
The Atlantis on this part of the British coast is the land of Lyonnesse. The low sea levels that allowed Doggerland to flourish, in the area that is now the North Sea, would have exposed land between the Scilly Isles and Cornwall. The rising sea levels on the islands today are just the end of a very long process that has seen the Scilly’s change from high peaks in a vast landscape to low lying islands. Legends abound.