L'Atalante basin

The L'Atalante basin is a very salty 'lake' at the bottom of the Mediterranean Sea.
It is about 190 km (~120 miles) west of the island of Crete. It is called a 'lake' because, though it is under more than two miles of water, it is almost completely separate from the sea above it. Its huge salt content prevents the waters mixing. Sometimes these lakes are called "brine pools". They are found in many oceans; there is one in the Gulf of Mexico.
How it formed.
L'Atalante is only one of several such lakes underneath the normal waters of the Mediterranean. When the Strait of Gibraltar was closed, about six million years ago, the Mediterranean dried up and became a huge salt lake, rather like the Dead Sea and the Great Salt Lake are today. Then it dried out completely, leaving an enormous amount of salt. When the Straight of Gibraltar opened again about five million years ago, water from the Atlantic Ocean rushed in to form the Mediterranean we see today. Even today, the Atlantic feeds the Mediterranean with water, because the evaporation from the 'Med' is greater than is provided by its rivers. With its huge supply of salt underneath, and the constant evaporation, the Med is a high salt sea compared with the Atlantic.
The L'Atalanta lake and its neighbours the Urania and Discovery lakes are about 35,000 years old. They were formed by salt deposits dissolving out of the Mediterranean Ridge and collecting in deep depressions at the bottom of the Med. L'Atalante is the smallest of the three. Its surface begins at about below sea level.
Salt content.
L'Atalante's salinity (salt content) is near saturation, at 36.5%, about eight times more than seawater. This prevents the water in the brine pool from mixing with the 'normal' seawater above. Without mixing, oxygen from above can't get into the brine pool. The basin's water is completely anoxic: it has no oxygen.
Between the normal water and the brine is a short (1.5 metres) boundary zone called a halocline. The salt content rises sharply there, and oxygen reduces towards zero. This halocline between the seawater above and the brine below has many bacterial and archaeal cells: they are chemoautotrophs, which feed on ammonia from the brine but cannot function without some oxygen. Certain methane oxidizers and haloarchaea are found only in the halocline.
Life without oxygen.
Below, in the brine, the number of cells is much lower than in the halocline. Extremophiles are most common, including some of the deep-sea hydrothermal vent archaea and bacteria. Eukaryotes are also found in l'Atalante, mostly ciliates (45%), dinoflagellates (21%) and choanoflagellata (10%).p360–381
The dark grey anoxic sediments at the bottom of L'Atalante lake are covered with a loose black layer. Microbes found in the sediments are almost all (90%) various species of "Bacillus". In 2010, three metazoan species, all in the Loricifera phylum, were discovered living in the sediment, the first multicellular lifeforms known to live entirely without oxygen.