What zone is the coldest and densest zone, deep below the surface of the lake.

This is the bottom layer, called the hypolimnion. Water in this layer usually has different properties to shallower layers. Colder and more saline (salty) water tends to sink, so the hypolimnion contains colder and saltier water. In addition, this layer also tends to contain less oxygen, and conditions can be anoxic. There is also more pressure at these depths, and organisms living in this layer have adapted to these pressures. Finally, biological material from the shallower layers tends to sink to the hypolimnion. During summer, when the surface layers are heated, temperature acts to strengthen the separation between the different layers in the lake. During winter, these temperature differences are less, and mixing between layers can occur.