The surface of a lake is frozen in severe winter, but the water at its bottom is still liquid. What is the reason ?

The survival of liquid water at the bottom of a frozen lake is primarily due to the anomalous expansion of water. Unlike most liquids, water reaches its maximum density at 4°C. As a lake cools during winter, the surface water becomes denser and sinks until the entire water body reaches 4°C. Upon further cooling below 4°C, the surface water becomes less dense and remains at the top until it freezes at 0°C. This creates a stable thermal stratification where the densest water (at 4°C) remains at the bottom, while the lighter ice and colder water float above [1]. Additionally, ice acts as a poor conductor of heat, which further insulates the liquid water below from the freezing atmospheric temperatures. This unique physical property ensures that deep lakes do not freeze solid, preserving aquatic life.

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 33: Ocean temperature and salinity > General Behaviour > p. 517