A thermocline is a boundary layer in bodies of water.
Unlike other fluids water's does not continue to get more dense as it gets colder. Fresh water, at normal atmospheric pressure, is most dense at 4 degrees Celsius.
One consequence of water's strange density is that solid water -- ice -- is less dense than water, and floats.
Another consquence is that deep lakes, located in areas that get cold enough in Winter where the temperature drops below 4 degrees long enough for the lake to cool develop a boundary layer separating a dense deep cool layer.
During the Spring, and the Fall, there is a period when the entire lake is at 4 degrees Celsius. During this period the water in the lake mixes, allowing water with dissolved oxygen to be mixed to the bottom of the lake.
In the fall, as the lake continues to cool below 4 degrees, the water at the surface of the lake becomes less dense and floats on the thermocline. Oxygen can't get down below the thermocline, until the water mixes again, in the Spring, when temperature at the surface of the lake rises to 4 degrees again, and all the lake water can mix again.
As the water at the surface of the lake warms, it becomes less dense, and it floats on the deep cold layer.
If the lake is deep enough it can retain a deep layer until the fall mixing.
With large enough deep lakes it is possible for cities to employ deep lake water cooling.
Deep tropical lakes can represent a very serious danger to nearby residents. Gases escaping from seismic activity, or gas generated from anaerobic biological activity can become trapped in the lakes' deep layers. The water at deep layers is under pressure. Water at high pressure can absorb high volumes of dissolved gas.
The danger to nearby residents arises when seismic activity, land-slides, or other accidents, cause the lake water to mix. If the deep lake water comes to the surface it is no longer under pressure and the dissolved gases come out of solution.
Gases like carbon dioxide are denser than regular air. When they come out of solution they can suffocate nearby residents, their domestic animals, and surrounding wildlife.
Thermocline and antisubmarine warfare
In antisubmarine warfare, the thermocline is often called the "layer", since it reflects the sound waves of active sonar. Submarines will try to hide below the layer, and surface or air antisubmarine platforms will try to get below the layer with a "dipping" sonar on a cable, or possibly with expendable sonar devices dropped through the layer.
In some tactical situations, however, the submarine cannot stay below the layer and carry out a mission because passive acoustic sensors may not be able to detect targets above the layer.