The Sea Demons' Excavation Plan

As to what exactly the "big squid" was, the sea demons actually couldn't explain it either.

About the forced-landing in ancient times, many sea demons' memories of it were already fuzzy. Although they were ageless and immortal elemental creatures, they couldn't remember incidents that were too old that clearly. However, the only thing the sea demons could be sure of was that the "big squid" at the bottom of the sea had already existed since the time they were forced to land in this world that had landmasses.