Of old, humans lived in Avalon, in the midst of the western ocean. Avalon was beautiful and salubrious, and humanity knew naught of hunger or want.
The more martially inclined speak of epic battles against dragons, elves and giants, fought with honour and in golden armaments, humanity ever victorious, and Avalon never breached.
Every culture and group tell of Avalon as the apex of virtue and civilization. For instance the Cargans tell of it as the ultimate ordered society, with marvelous roads and waterworks, while the Free Kingdoms portray it as a most chivalrous feudal society, with countless spired castles.
The Golden Age was ended, Avalon sank into the ocean. There are different traditions as to why?
Divine punishment: Humankind had grown decadent and sinful, and the gods eventually put an end to their errors by inonduating Avalon.
Divine caprice: The more fatalistic claim that the Gaea, goddess of the hoary chaos, simply sank the land, for no other reason than chaos itself.
Elfin evil: The elves wove their greatest magic ever, to eradicate their ancestral enemies, and then enslaved the survivors.
Human hubris: The men of Avalon discovered magic, and as a result, Avalon was sunk, whether from magics unleashed or the punishment of the gods.
These explanations are often mixed and matched. The Confederates see Avalon as the perfect magocracy, and claim that elfin sabotage of human sorceries was the cause of its sinking.
Whatever the cause, Avalon sank amidst earthquakes and giant waves, its cities and riches were lost, and most of its inhabitants drowned.
The men of the 6th Age universally seem to regard themselves as descendants of people who fled Avalon before or during the sinking. Perhaps the Atraharsites takes this the furthest, naming themselves after a mythical boatbuilder of Avalon.
Most cultures claim to be rebuilding Avalon.
Humans in general and atraharsites especially, tend to refer to primitive humans as antediluvians "those who were here before the flood".
Avalon, while sunk, persists as a mythical place. It is said that great heroes are brought by angels (valkyries, etc.) to Avalon at their time of death, and Avalon and Elyseum (paradise) are entwined, in the minds of common folks at least.
Sailors' myths speak of Fiddler's Green or The Islands of the Blessed, islands in the Ocean of Lost Souls which are like Avalon, and may be remains of it.
Less romantic speculations claim that since Avalon must have had mountains like any other continent, parts of it may still remain above sea level, though probably not in such a paradisical form as Fiddler's Green.
Elves are said to be even more brusque than usual when it comes to the question of Avalon. This is thought to be because they hate to think of the time when humans were supreme, and/or because they fear that their complicity in its sinking may be held against them.