The ocean floor hides many secrets and curiosities. Such as rivers and lakes. On the bottom of the ocean. Undersea rivers and lakes often form in straits, near freshwater springs, in sunken caves and similar places where local anomalies encourage the segregation of more and less salty water.
Pictured: Ancient Mayans would often throw human sacrifices at this location believing there was a entrance to the afterlife. Little did they know what was actually on the bottom.
Pictured: The shores of an undersea lake near the Yucatán peninsula. Note the foggy waves silently crashing into „the shore“.
Water with more salt sinks near the bottom and forms there mystical sights. The phenomena is actually pretty large: the sixth largest river on Earth is an undersea one, in the Black Sea. The Mediterranean Sea, already saltier than average, due to its isolation, crawls on the sea bed into the Black Sea which is relatively sweet due to (normal) rivers supplying it with freshwater. That river is 350 times larger than the much more famous Thames river in London, but far less explored.
Pictured: A double swim for double fun.
Undersea rivers and lakes have their own streams, waterfalls, even flooding of nearby fields and forests of kelp that can annihilate life unaccustomed to such levels of salt. Of course, they too have their own ecosystems, mostly made up from microorganisms, worms and shrimps which can tolerate living in a world with such extreme levels of salt and in almost total darkness.
Pictured: The Cenote Angelita sunken cave complex. On the surface of the water there is almost no salt and the natives use it for drinking.
They are so powerful and ancient that there are even canyons on the sea floor carved by undersea rivers.
Pictured: The river beneath the Black Sea in contrast color for easier observing. Picture taken using some kind of science thingy.
Pictured: An undersea lake from afar.
And finally, a similar phenomena, but vertically instead of horizontally
Some interesting videos about this subject on YouTube: