It is likely that until relatively recent times the islands were much larger and perhaps joined together into one island named Ennor. Rising sea levels flooded the central plain around 400–500 AD, forming the current 55 islands and islets, if an island is defined as “land surrounded by water at high tide and supporting land vegetation”. The word Ennor is a contraction of the Old Cornish En Noer (Moer, mutated to Noer), meaning ‘the land' or the ‘great island’. […]
The islands may correspond to the Cassiterides (‘Tin Isles’) believed by some to have been visited by the Phoenicians, and mentioned by the Greeks. However, the archipelago itself does not contain much tin.
Source: Isles of Scilly – Wikipedia