Hypolimnion

The hypolimnion or under lake is the dense, bottom layer of water in a thermally-stratified lake. The word "hypolimnion" is derived from Ancient Greek: λιμνίον, romanized: limníon, lit. 'lake'. It is the layer that lies below the thermocline.

Typically the hypolimnion is the coldest layer of a lake in summer, and the warmest layer during winter. In deep, temperate lakes, the bottom-most waters of the hypolimnion are typically close to 4 °C throughout the year. The hypolimnion may be much warmer in lakes at warmer latitudes. Being at depth, it is isolated from surface wind-mixing during summer, and usually receives insufficient irradiance (light) for photosynthesis to occur.