Underground Lake

This large underground lake was found deeper within Greg's Mine and seemed to be filled with salt water from the ocean. A sandy beach encircled about half of the huge lake and the outcroppings of a temple (Underwater Sahuagin temple) can be seen poking out above the water level.
On the nearby beach the party encountered the wounded and nesting Cragyss whou was using the former ship of Trefal as a shell. She was willing to trade shells if the party could find another or reclaim her old one from a pair of sea serpents.
She informed the party of it, some entity that the sea serpents served and radiated anger, madness and power. It was taken from the lake and enclosed somewhere by stone.
Further along the beach they also found the skeleton of a sailor, from whom they recovered a mostly gone note (Waterlogged Note).