The Lost Kingdom

To the Tanishites, it's the land beyond the arch at The Ascending Ground. The Winged Ones departed through it during The First Reckoning and never returned. When a girl is sacrificed to Rekeh, she travels there, too, and meets the Creator in a land of plenty.
To the Vishim People of the desert, it's a great underground city where the Winged Ones descended after losing their wings.
The Baan people of Taalora speak of a small continent floating in the sky, full of forests and fields and great cities of winged people.
Almost every culture has a similar myth. Anthropologist Shalov Yarolezåzus, writing in 114 BR, had this to say on the subject:

There are documented myths in almost all surveyed societies that include some sort of promised land that is unreachable by man. Certainly readers will be familiar with our own stories of the Land Under Waves. Though this story of a sunken country is considered a nursery story by many, the prevalence of related myths lends some credence to the idea that these stories have some basis in history.

The core facts that every story includes are that there is some landmass unreachable by man, and that the Winged Ones of old retreated there after the Great Catastrophe. It's possible that there was some mass exodus in the distant past, and that various societies explained it in their own ways. If these myths do describe an historical exodus, it stands to reason that this 'Lost Kingdom' exists in in a physical state somewhere in the world.

Yarolezåzus led an expedition to find his alleged Lost Kingdom in 109 BR, sailing east into the Rheic Ocean. The expedition never returned.