A sea of dense water under the ocean is...?

Question

It might have been on the Blue Planet series I saw this covered, but I can't recall the details. It was near a cold seep that undersea explorers discovered an "underwater sea". The water in it was more dense than the surrounding water, so the dense water sank and formed its own distinct layer, complete with a rippling surface and shoreline. What I want to clarify is what causes this layer to be so heavy? Does it have significantly more salt dissolved in it? Or is it the result of other chemicals in the water? What makes the sunken sea different from the surrounding water?

Right, I know temperature makes a difference, but the case I'm describing is extreme. The exploration sub that literally ran into it actually bounced off the surface there was such a stark difference in density. Touching it resulted in ripples and splashes as though it were an ordinary water surface touching air rather than more water. So far I'm agreeing that it's high salinity that's the cause of this phenomenon.

Ah ha! I found a picture of the lake's "shoreline".
http://www.gozir.com/logman/wp-content/uploads/2007/07/underwater-lake.jpg

You can see the distinct surface of the liquid and the organisms crowding the "shore". Also, it might not be water at all, but possibly liquid carbon dioxide. I'm not sure if that's the same as this though.

Answer 1

The cases I know of where there are distinct pools are caused by high salinity. Groundwater passing through evaporite deposits gets very saline, and when it seeps into the ocean, it migrates into basins. I think the earliest discoveries of this phenomenon was in the Red Sea in the mid-1960s. For some reason Degens and Ross are the names that appear in my head in association with the reports on this.

Answer 2

Temperature is the most likely thing. The colder water seeps into an area without a lot of currents and movement, and it tends to stay put and not mix very much. And colder liquid water will be a little denser than warmer, so it will stay on the bottom. (unless it is cold enough to freeze - then it becomes less dense and floats.)
Submarines have to deal with that. There are boundary layers between colder and warmer layers, and sonar waves tend to bounce off them rather than penetrate through, as if they were solid boundaries.

Answer 3

Stratification in large bodies of water due
to either or both temperature and salinity
differences are not uncommon.
It is density difference that drives the gulf stream,
the labrador and humbolt currents.