There were still many sections of maps that were never filled in. It was assumed that there were things there, but very few ever returned. Those that did often drank themselves to death in a short few month.
In this case a large land mass that had impacted the center of the old ocean, and within that was the fragments of a monster that refused to accept death. Most of it was buried under the earth, one of the few blatant descents of the goddess Nyx. As she had yet such a stranglehold on the earth yet, the lonely old god that survived took the opportunity to bind some of that earth to an undersea volcano.
It erupted constantly for almost three years forming a large set of plain islands before it was sprinkled across all the land. While it wasn't well known, those that did knew it at the land of the fallen.
