When humans first arrived on the 3rd Floor of the Abyss, they were terrified and desperately tried to find a way out.
But the 3rd Floor of the Abyss was not a place one could come and go from as they pleased.
After countless failed attempts, they could only accept their circumstances. They found a relatively safe island and settled down.
Then, the humans began to gradually explore this strange sea.
After encountering all sorts of strange phenomena, they came to believe this was the Immortal Realm, a world where Immortals lived. That, they thought, was why so many wondrous things happened.
It wasn't until they encountered a race they could communicate with that the humans learned the basics about the Abyss.
That race was called the Chali.
The Chali Race's settlement wasn't far from the human island; it was only a half-day's voyage away.
