Deep below the sea exists underwater lakes and rivers that boast their own mini ecosystems.
Seafloor lakes are actually brine pools. Brine is water with extremely high concentrations of salt, so high that the mixture is heavier than water and lies underneath the normal sea salted water.
Brine pools are formed by salt tectonics but in the Mexican Gulf, specifically, during the Jurassic period, the seas were shallow and soon dried out to form a think layer of salt up to 8km thick. As water returned to the area, it filled again with normal sea water and the super-saline layer that had been covered with sediment, and preserved to became an underwater lake.
During an expedition in the Gulf of Mexico in 2007, Natural Marine Sanctuaries captured images of a brine channel at the base of East Flower Garden Bank. Hypersaline water flowing from under the sea floor created a concentrated brine lake and river measuring about 10 inches deep.
Because of the high salinity of seafloor lakes, nothing can live in the brine but bacteria, and a few other creatures like molluscs or shrimp can live near it.
Source: Yahoo! Green
Explore a seafloor lake with David Attenborough:
During the Deep Slope 2006 Exploration, the Alvin submersible dive scientist observed an extraordinary sight. Listen to scientist Harry Roberts, Mandy Joye, and Gavin Eppard as they witness an underwater wave pass across the surface of a Brine Lake, at the interface between the ‘lake’ and overlying ocean: