Gadvia

Hundreds of years ago, scientists discovered Blue Crystal, a new form of energy. This resulted in a rapid rise in technological level, with humanity setting foot in the universe and even the deep sea. However, the overuse of crystal energy awakened the primordial gods of the Earth, they created a flood that submerged all the continents, and sank all advanced human technologies to the deep sea, nearly erase all civilizations.

Remnants of human survivors rebuilt their homes on deserted islands and are once again believers in the gods. One day, the waves washed something other than debris ashore, a girl from the depths of the sea that calls herself Limnoreia is found on the beach. Turns out this curious girl is sent to the surface with the heavy burden of finding a way to save the increasingly resource-depleted deep-sea sanctuary where she came from...