The story the mages tell says that, in the dawn of humanity, certain dreaming humans were drawn to an island at the centre of the world. In the centre of that island was a spire of rock that reached into the heavens. By fasting and meditating in caves at the base of the spire, among the crystal bones of vanished dragons, the dreamers Awakened, returned to the Mundane World, but with their souls linked forever to the Supernal World.
They were the first mages.
In time, they became mighty rulers of much of the world, but aspired to be more, and so built a golden ladder climbing from the top of the spire into the Supernal World. The wizard kings of Atlantis climbed up into the Supernal World, become as gods.
Others feared what they would do with such vast power and pride, and pursued them, battling them in a mystical war at the top of the golden ladder, until the ladder shattered, and the world broke.
Atlantis was drowned in the seas, and the Abyss opened between the Mundane World and the Supernal World, bringing with it the Quiescence. Those mages who survived in the Mundane World saw their powers diminish and fade, and no new Awakenings occurred, until the first Watchtower was established and sent its siren call across the Abyss.
This story may or may not be true. It is certainly symbolic of what happened, and to magical thinking, that’s the same thing. But different mages put a different spin on the story.
Some link it to the deluge myths so prevalent in many cultures, some to the multiple successive world cosmologies of the Navajo and the Maya. Others liken it to Promethean tales, the bringers of forbidden knowledge being punished by the gods, or to lost paradise stories, such as the Biblical Eden. Because of the nature of the change that the event caused, the truth is a malleable thing, based on perception and symbolic understanding.
For the sake of convenience and tradition, mages long ago agreed to use the name Atlantis for this crux of mystery. Most understand that it doesn’t mean only what it seems to mean, but is more of a code word for the concept.