The sight of a lake at the bottom of the ocean is simply amazing. Here is some info from Wikipedia
about these Brine Pools:
Brine pools are large areas of brine on the ocean basin. These pools are concentrations of water having an extremely high salinity as compared to the surrounding ocean, caused by the motion of large salt deposits through salt tectonics. Methane is also in high concentration in these areas, providing energy by the process of chemosynthesis to creatures which live in the pool's vicinity. These creatures are often extremophiles.
Brine pools are considered "lakes" within the ocean by many oceanographers. This is due to the high salinity of the water in the pool, which prevents water of lower salinity from entering and creates a distinct surface and shoreline for the pool.