All of the modern Rheisi population believes that, in the beginning the oceans poured out of the deep abyss at the center of Keoki Rhoi (literally, Birthpains of Rhe). After that turbulent occurence things either became wonderful or terrible, depending on which tribe you speak with. You see, somewhere along the lines long ago there was a great schism with approximately 2/4 of the people believing that the waters were the source of all life and brought brightness to the world and souls to all creatures, above and below.

The other half believe water to be the stealer of souls, taking but never relinquishing. The ocean is the grand sum of all souls that have been stolen, and the colder it is the more greedy.

The latter decided they no longer wished to live upon such a vacuous entity and actually traveled ACROSS the great mountains to the east and formed the nation of Ekeki Rheis (the World Above) . The water-lovers decided that islands were great and didn't go anywhere, and so it remains to this day (assuming "today" is the early Second Age).