What are brine pools on the ocean floor?

The answer

Brine pools are bodies of water on the ocean floor that are so dense with dissolved salt (four to five times saltier than surrounding seawater) that they form a distinct boundary resembling a shoreline. These pools are often toxic to most marine life due to extreme salinity and dissolved methane or hydrogen sulfide, creating a so-called 'lake of death.' However, specialized extremophile bacteria and certain mussels thrive at the edges. Some of the best-studied brine pools sit at the bottom of the Gulf of Mexico and the Red Sea.