In lakes, the lake bottom sediments free of vegetation that lie below the deep water Pelagial Zone.

The profundal zone is a deep zone of a body of water, such as an ocean or a lake, located below the range of effective light penetration. This is typically below the thermocline, the vertical zone in the water through which temperature drops rapidly. The lack of light in the profundal zone determines the type of biological community that can live in this region, which is distinctly different from the community in the overlying waters.