Let us examine the mythical "sinking" islands: Yokuda, Thras, and Aldmeris. Each island was the ancestral home of its resident race, and in all three cases, foes or fate destroyed the island as punishment for some act of hubris. In the case of the Redguards, foolish sword-singers sundered the Yokudan Isles with a forbidden sword stroke. The warriors of the All-Flags Navy drove the Sload and their island of Thras into the sea as punishment for the Thrassian Plague. And our forebears, the Aldmer, fled the Isle of Aldmeris to avoid some mysterious calamity—likely the result of our fall from Aedric grace.
Now, a novice historian would likely take these tales at face value. "If multiple histories say the island sank, it must have sunk!" But I entreat you to look deeper. Could it be that the "sinking island" is not a literal event, but rather, a metaphorical one?
Thras, Yokuda, and Aldmeris are much more than simple landmasses; they are societal symbols—avatars for a cultural identity lost in time. So stories about the sundering or sinking of these islands may be a trick of the light—a poet's attempt at explaining the pain of forgotten origins. Did an entire continent drown as a result of a sword stroke? Did our ancestors travel to Summerset from a mystical half-Aedric isle? I think not. These lost islands rest at the fault-line between fact and parable. There is truth in those tales, certainly, but the true historian knows that not all truth is literal.
--Excerpt from The Ubiquitous Sinking Isle