The temperature of water at the bottom of a lake whose upper surface has frozen to ice would be around

The temperature of water at the bottom of a frozen lake is around 4°C due to the anomalous expansion of water. Unlike most substances, water reaches its maximum density at 4°C. As a lake cools, the surface water becomes denser and sinks until the entire water column reaches 4°C. Upon further cooling below 4°C, water begins to expand and its density decreases, causing the colder water (0°C to 3°C) and eventually ice to float on the surface. This creates an inverse stratification where the densest, warmest water (4°C) remains at the bottom, while the lighter ice forms an insulating layer at the top. This unique physical property prevents lakes from freezing solid, thereby preserving aquatic life during winter.