Surely you may have seen, in cartoons or documentaries, people fishing in ice holes. But as we have seen, liquids dilate in the same way as solids, ie evenly, so how is it possible that there is liquid water under the ice sheets at or below 0 ° C?
This phenomenon occurs due to what we call anomalous dilation water, because at a temperature between 0 ° C and 4 ° C there is an inverse phenomenon to the natural and expected. In this temperature range the water, when cooled, undergoes an expansion in its volume, and when heated, a reduction. This is what allows life to exist in the water in extremely cold places, such as the North Pole.
The uppermost layer of water in lakes, seas and rivers cools due to the freezing air, increasing its specific mass and making it heavier, so a convection process occurs until all the water reaches a temperature of 4 ° C, after this freezing occurs from the surface to the bottom.
We can represent the behavior of water volume as a function of temperature:
As you can see, the smallest volume for water happens at 4 ° C.