A link between eras.

...Deep, even deeper than the dungeon, lies a vast sea. This sea is filled with many secrets. It was here that the inhabitants of Thallia found refuge during the Cataclysm. Many of them were able to adapt to the new conditions and live underwater, especially since they were amphibians. Some of them, in particular the hydras, decided to rise higher and ended up with the dark elves. From them, we learned about the underwater sea.