At the bottom of the sea floor water seeps up through a thick later of salt, creating REALLY salty dense water. The water dissolves the salt layer, forming depressions aka basins that form lakes. Really salty water (brine) form lakes and rivers due to high density and being heavier than seawater. Literally a lake in the sea. Brine lakes can even have waves and tides!
Due to the hyper salinity (high salt), the brine is lethal. Only bacteria and other microorganisms can live in it. However, mussels live at the edge! Under normal conditions, mussels eat algae, but we are talking about the bottom of the sea floor where there is no sunlight or algae. These mussels have a symbiotic relationship with bacteria! The bacteria process methane that seeps up and provides energy to the mussel.
Mussel beds provide habitat to other animals like eels. Eels are able to go into the underwater lake, but only for a short time. If they stay too long under under water, the brine becomes toxic and they die